grub_machine_get_bootlocation and partitions

[Andrei Borzenkov]

If partition information is actually present, it results in bogus and
invalid device name like

hd0,1

This device cannot be opened as it requires partmap name.

One obvious case when partition *is* passed to GRUB is multiboot. I
suppose nobody ever noticed it because standard grub-install generates
fixed partition part.

We probably can fallback to probing partmap on current level in this
case, except it means more code again. But current situation is rather
useless as is.