qemu-iotests: Test bdrv_close while AIO is in flight

If the BlockDriverState is closed/freed without draining the AIO
requests first, the request coroutines may work on invalid data and file
descriptors or have some dangling pointers that cause segfaults.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
