ceph-disk: is_mpath predicate for multipath devices

ceph-disk: is_mpath predicate for multipath devices

The is_mpath predicate returns True if a device is managed by
multipath. It is based on the devicemapper uuid content which is
expected to always contain the mpath- string to identify the multipath
subsystem.

The block_path helper is added to convert the path to a device to the
/sys directory that describes it. It uses the major and minor number
instead of the device name because it is more reliable. The rationale
including an actual example is added as a comment for future
maintainers.

Signed-off-by: Loic Dachary <ldachary@redhat.com>