pve7to8: check for proper grub meta-package for bootmode

pve7to8: check for proper grub meta-package for bootmode

This should catch installations from our ISO on non-ZFS in uefi mode,
which won't get the updated grub efi binary installed upon upgrade,
because grub-pc is installed instead of grub-efi-amd64.

Adding this to pve7to8 should make this even more visible, than the
corresponding patch for promxox-kernel-helper (warnings printed during
regular package upgrades might be overlooked more easily than
a yellow line in the major upgrade checkscript)

The if/else order was chosen to limit the nesting level of the long
messages.

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>