iotests: Test commit with iothreads and ongoing I/O

iotests: Test commit with iothreads and ongoing I/O

This tests exercises graph locking, draining, and graph modifications
with AioContext switches a lot. Amongst others, it serves as a
regression test for bdrv_graph_wrlock() deadlocking because it is called
with a locked AioContext and for AioContext handling in the NBD server.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20230517152834.277483-4-kwolf@redhat.com>
Tested-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>