C'est tout à fait ça, j'ai modifié "à la main" le chemin des machines dans "VirtualBox.xml" et le "dossier par défaut des machines" dans le gestionnaire.
Contenu du fichier "VirtualBox.xml" :
<?xml version="1.0"?>
<!--
** DO NOT EDIT THIS FILE.
** If you make changes to this file while any VirtualBox related application
** is running, your changes will be overwritten later, without taking effect.
** Use VBoxManage or the VirtualBox Manager GUI to make changes.
-->
<VirtualBox xmlns="http://www.virtualbox.org/" version="1.12-linux">
<Global>
<ExtraData>
<ExtraDataItem name="GUI/Details/Elements" value="generalClosed,systemClosed,preview,displayClosed,storageClosed,audioClosed,networkClosed,usb,sharedFolders,description"/>
<ExtraDataItem name="GUI/DetailsPageBoxes" value="general,system,preview,display,storage,audio,network,usb,sharedFolders,description"/>
<ExtraDataItem name="GUI/GroupDefinitions/" value="m=8b908519-f25c-4676-abc1-82e5e58fdf54,m=6a41bdb3-a4a3-4a91-83a2-74761c9be9ec,m=1b36c9aa-1858-4e76-a63b-52361407d971,m=69040233-ea84-4c26-83e1-f5330c79a8d6,m=aa91d8ea-5257-42bc-b3cc-e8c32c251abd"/>
<ExtraDataItem name="GUI/HideDescriptionForWizards" value="NewVM,ImportAppliance"/>
<ExtraDataItem name="GUI/HostScreenSaverDisabled" value="false"/>
<ExtraDataItem name="GUI/Input/AutoCapture" value="true"/>
<ExtraDataItem name="GUI/Input/HostKeyCombination" value="65508"/>
<ExtraDataItem name="GUI/LanguageID" value="fr"/>
<ExtraDataItem name="GUI/LastItemSelected" value="m=3DSMAX"/>
<ExtraDataItem name="GUI/LastWindowPosition" value="955,108,957,843"/>
<ExtraDataItem name="GUI/LogWindowGeometry" value="656,265,1330,792"/>
<ExtraDataItem name="GUI/RecentFolderCD" value="/home/tabou/NAVETTE_8"/>
<ExtraDataItem name="GUI/RecentFolderFD" value="/media/SaveDisk/Iso/Win98"/>
<ExtraDataItem name="GUI/RecentFolderHD" value="/media/tabou/TOSHIBA EXT1/VirtualBox VMs/Nouveau groupe/windows 10"/>
<ExtraDataItem name="GUI/RecentListCD" value="/home/tabou/NAVETTE_8/VBoxGuestAdditions_6.0.6.iso,/media/tabou/TOSHIBA EXT/Inateck/isos/Autodesk 3ds Max 2017 (x64) + Keygen [SadeemPC].iso,/media/tabou/TOSHIBA EXT/Inateck/isos/fr_windows_7_ultimate_with_sp1_x64_dvd_u_677299.iso,/media/tabou/TOSHIBA EXT/Inateck/isos/AIO Windows 8 RTM 32-64Bits.mundomanuales.com.iso,/media/tabou/TOSHIBA EXT/Inateck/isos/670234-A27 - OS Windows 7 SP1 Pro x64.iso"/>
<ExtraDataItem name="GUI/RecentListFD" value="/media/SaveDisk/Iso/Win98/WIN98SEC.IMG;"/>
<ExtraDataItem name="GUI/RecentListHD" value="/media/tabou/TOSHIBA EXT1/VirtualBox VMs/Nouveau groupe/windows 10/W10.vdi,/media/tabou/TOSHIBA EXT1/VirtualBox VMs/Windows 8/NewVirtualDisk1.vdi,/media/tabou/TOSHIBA EXT/VirtualBox VMs/Windows 8/Coffre_W8_2.vdi,/media/tabou/TOSHIBA EXT1/VirtualBox VMs/Windows 8/Coffre_W8.vdi,/media/tabou/TOSHIBA EXT1/VirtualBox VMs/Windows Vista/Vista 32.vdi"/>
<ExtraDataItem name="GUI/SplitterSizes" value="266,687"/>
<ExtraDataItem name="GUI/Statusbar" value="false"/>
<ExtraDataItem name="GUI/SuppressMessages" value="showRuntimeError.warning.DvdOrFloppyImageInaccessible"/>
<ExtraDataItem name="GUI/Toolbar/GlobalTools/Order" value="VirtualMedia"/>
<ExtraDataItem name="GUI/Toolbar/MachineTools/Order" value="Details"/>
<ExtraDataItem name="GUI/Tools/LastItemsSelected" value="Welcome,Details"/>
<ExtraDataItem name="GUI/TrayIcon/Enabled" value="false"/>
<ExtraDataItem name="GUI/UpdateCheckCount" value="205"/>
<ExtraDataItem name="GUI/UpdateDate" value="1 d, 2021-02-20, stable, 6.1.18"/>
<ExtraDataItem name="GUI/VirtualMediaManager/Details/Expanded" value="true"/>
</ExtraData>
<MachineRegistry>
<MachineEntry uuid="{69040233-ea84-4c26-83e1-f5330c79a8d6}" src="/media/tabou/TOSHIBA EXT1/VirtualBox VMs/Windows Vista/Windows Vista.vbox"/>
<MachineEntry uuid="{1b36c9aa-1858-4e76-a63b-52361407d971}" src="/media/tabou/TOSHIBA EXT1/VirtualBox VMs/Windows 8/Windows 8.vbox"/>
<MachineEntry uuid="{aa91d8ea-5257-42bc-b3cc-e8c32c251abd}" src="/media/tabou/TOSHIBA EXT1/VirtualBox VMs/Nouveau groupe/windows 10/windows 10.vbox"/>
<MachineEntry uuid="{6a41bdb3-a4a3-4a91-83a2-74761c9be9ec}" src="/home/tabou/VirtualBox VMs/El Capitan/El Capitan.vbox"/>
<MachineEntry uuid="{8b908519-f25c-4676-abc1-82e5e58fdf54}" src="/media/tabou/TOSHIBA EXT1/VirtualBox VMs/3DSMAX/3DSMAX.vbox"/>
</MachineRegistry>
<NetserviceRegistry>
<DHCPServers>
<DHCPServer networkName="HostInterfaceNetworking-vboxnet0" IPAddress="192.168.56.100" networkMask="255.255.255.0" lowerIP="192.168.56.101" upperIP="192.168.56.254" enabled="1"/>
</DHCPServers>
</NetserviceRegistry>
<SystemProperties defaultMachineFolder="/media/tabou/TOSHIBA EXT1/VirtualBox VMs" defaultHardDiskFormat="VDI" VRDEAuthLibrary="VBoxAuth" webServiceAuthLibrary="VBoxAuth" LogHistoryCount="3" exclusiveHwVirt="true"/>
<USBDeviceFilters/>
</Global>
</VirtualBox>