Hi,
Qubes must be installed in UEFI, because legacy is not supported for
internal disk. Booting USB in legacy mode results in legacy mode of internal disk.
To do this you have to edit second partition of USB key:
Add nouveau.modeset=0 to the kernel parameters in
/boot/efi/EFI/qubes/BOOT64X.cfg in qubes-verbose
I also commented noexitboot and mapbs.
More info:
https://www.qubes-os.org/doc/uefi-troubleshooting/Jan Fabo