Attach the qubes partion to your computer. In this example the partition is at /dev/xvdi however it may be at a subaddress on the device depending on how you mounted the storage device.
Once the device is available run:
udiskctl unlock -b /dev/xvdi
Enter your Qubes passphrase.
mount -o ro /dev/qubes_dom0-root /mnt
VMs are stored at /mnt/var/lib/qubes/
In this example we are going to access an AppVM called self
Mount the vm private.img
sudo mkdir /self sudo mount -o ro /mnt/var/lib/qubes/appvms/self/private.img /self
Read and copy files as needed from the /self directory