osdep/hurd/getroot: Use "part:" qualifier

osdep/hurd/getroot: Use "part:" qualifier

When using userland drivers such as rumpdisk, we'd rather make ext2fs use
parted-based libstore partitioning support. That can be used for kernelland
drivers as well, so we can just make GRUB always use the "part:" qualifier
to switch ext2fs to it.

grub_util_find_hurd_root_device() then has to understand this syntax and
translate it into the /dev/ entry name.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>