discover: Handle BTRFS root subvolumes

During install some distributions[0] will create subvolumes when formatting
the root filesystem with BTRFS. In particular this can mean that
bootloader config files will appear (in the case of GRUB) under
/var/petitboot/mnt/dev/$device/@/boot/grub/
rather than the expected
/var/petitboot/mnt/dev/$device/boot/grub/

If this is the case, perform all file operations from the parser
relative to this subvolume rather than the mount point. At the moment
this only supports the trivial case where the subvolume name for root is
blank (ie. '@').

[0] In particular, Ubuntu from at least 14.04

Signed-off-by: Sam Mendoza-Jonas <sam@mendozajonas.com>