skyring: Delete storage entity only if no block devices
skyring: Delete storage entity only if no block devices

Added logic to error out with message that storage cannnot
be deleted if there are block devices backed by the
storage. First the block devices should be deleted and then
only storage can be deleted.

Change-Id: Ic6cb37d8b96a2c4bbc16b23cc9a36cd45f6805eb
Signed-off-by: Shubhendu Tripathi <abf833e62503b33bb0dfae62bb381d87d91f8de1@redhat.com>

skyrings/skyring,skyrings/skyring,skyrings/skyring,skyrings/skyring