https://bugzilla.redhat.com/show_bug.cgi?id=1281710

Commit id '3c7590e0' added the flag to the rbd backend, but provided
no means via virsh to use the flag.  This patch adds a '--delete-snapshots'
option to both the "undefine" and "vol-delete" commands.

For "undefine", the flag is combined with the "--remove-all-storage" flag
in order to add the appropriate flag for the virStorageVolDelete call;
whereas, for the "vol-delete" command, just the flag is sufficient since
it's only operating on one volume.

Currently only supported for rbd backends.