Don't assume grub is on a separate boot filesystem

Don't assume grub is on a separate boot filesystem

Paths in grub.conf are relative to the filesystem containing it. grub parsing
currently assumes that it is on /boot, and will fail if it isn't, for example
because a guest only has a single partition.

This patch makes grub parsing work harder to work out what grub paths are
relative to. Firstly, it looks for a previous detected 'linux-grub' filesystem.
If this isn't found, it tries to work out which filesystem contains
/boot/grub/menu.lst and uses that.