rbd: match against whole disks on unmap

rbd: match against whole disks on unmap

Currently the way 'rbd unmap' translates a user-provided block device
into an rbd id is it matches the major number of the specified device
against /sys/bus/rbd/devices/<id>/major for each rbd mapping and
declares success on the first match.  This works for both entire disks
and partitions, because under the current device number allocation
scheme, each mapping means a new major number.

In preparation for support for single-major device number allocation
scheme, which would require matching both major and minor numbers, make
sure to always match against entire disk device numbers, by converting
the specified device major:minor pair into wholdedisk major:minor pair.
To achive that, use the libblkid library, which accomplishes this goal
by walking stable sysfs structures.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>