In qmp_transaction, assert that the BdrvActionOps to be used is actually
valid.

This assertion failing is very improbable, however, it might happen, if
a new TransactionActionKind is introduced "out of order" and the
actions[] array is not updated.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>