bdrv_try_set_aio_context() currently fails if a BlockBackend is attached
to a node because it doesn't implement the BdrvChildRole callbacks for
AioContext management.
We can allow changing the AioContext of monitor-owned BlockBackends as
long as no device is attached to them.
When setting the AioContext of the root node of a BlockBackend, we now
need to pass blk->root as an ignored child because we don't want the
root node to recursively call back into BlockBackend and execute
blk_do_set_aio_context() a second time.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
38475269d4