blockdev: check for BLOCK_OP_TYPE_INTERNAL_SNAPSHOT_DELETE

The BLOCK_OP_TYPE_INTERNAL_SNAPSHOT_DELETE op blocker exists but was
never used!  Let's fix that so snapshot delete can be blocked.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
