Grub scripts are very persistent in trying to use what is currently
mounted as /. Even if currently (TemplateVM) /dev/xvda3 is mounted
directly, all the configuration should use /dev/mapper/dmroot, to work
also in AppVM.
GRUB_DEVICE is used in various places as root device (including
constructing root= parameter in some versions). Force it to
/dev/mapper/dmroot
QubesOS/qubes-issues#6174