grub-install: Change test to use /dev/sda instead of /dev/vda.

The previous test for grub-install hard-coded /dev/vda (ie. assuming
virtio-blk instead of more modern virtio-scsi).

This changes the test to hard-code /dev/sda instead.  However this
change is still not correct since /dev/sda will be adjusted by block
device name translation in the call to grub-install, but not what is
written to /boot/grub/device.map.

Since we no longer support grub-install on Fedora, this won't affect
things, but Ubuntu still has a 'grub-install' command (although it's
actually from grub2).