blockdev: add x-blockdev-set-iothread force boolean

When a node is already associated with a BlockBackend the
x-blockdev-set-iothread command refuses to set the IOThread.  This is to
prevent accidentally changing the IOThread when the nodes are in use.

When the nodes are created with -drive they automatically get a
BlockBackend.  In that case we know nothing is using them yet and it's
safe to set the IOThread.  Add a force boolean to override the check.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20171207201320.19284-4-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
