test-bdrv-drain: Don't call bdrv_graph_wrlock() in coroutine context

test-bdrv-drain: Don't call bdrv_graph_wrlock() in coroutine context

AIO callbacks are effectively coroutine_mixed_fn. If AIO requests don't
return immediately, their callback is called from the request coroutine.
This means that in AIO callbacks, we can't call no_coroutine_fns such as
bdrv_graph_wrlock(). Unfortunately test-bdrv-drain does so.

Change the test to use a BH to drop out of coroutine context, and add
coroutine_mixed_fn and no_coroutine_fn markers to clarify the context
each function runs in.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20230929145157.45443-2-kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>