dev-mtp: rename x-root to rootdir

x-root was renamed as such owing to the experimental nature of the
property; the underlying filesystem semantics were undecided

Signed-off-by: Bandan Das <bsd@redhat.com>
Message-id: 20180720214020.22897-6-bsd@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>