devmapper: check for valid device abstraction in get_grub_dev

devmapper: check for valid device abstraction in get_grub_dev

This was lost when code was refactored. Patch restores previous behavior.

It is still not clear whether this is the right one. Due to the way we
detect DM abstraction, partitions on DM are skipped, we fall through to
generic detection which ends up in assuming parent device is BIOS disk.

It is useful to install GRUB on VM disk from the host. But it also means
that GRUB will mistakenly allow install on real system as well.

For now let's fix regression; future behavior needs to be discussed.

Closes: 45163