blockdev: check for BLOCK_OP_TYPE_INTERNAL_SNAPSHOT

The BLOCK_OP_TYPE_INTERNAL_SNAPSHOT op blocker exists but was never
used!  Let's fix that so internal snapshots can be blocked.

[Fixed s/external/internal/ typo as pointed out by Paolo Bonzini and Max
Reitz.
--Stefan]

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 1416566940-4430-5-git-send-email-stefanha@redhat.com
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
