Disallow snapshots of mirror segment types.

Disallow snapshots of mirror segment types.

Snapshots of RAID logical volumes are allowed (including "raid1").  However,
snapshots of "mirror" logical volumes has been disallowed due to unsolvable
issues inherent to the design.  The fact that mirroring (dm-raid1.c) must
stop all I/O as the result of a failure and wait for userspace intervention
can lead to a circular dependency if userspace is simultaneously waiting for
snapshots (on mirrors) to make an I/O update before proceeding.

Various snapshot on mirror tests have been removed as a result.