tests: avoid qemu_aio_flush() in test-thread-pool.c

We need to eliminate calls to qemu_aio_flush() since the function is
being removed.  Most callers will use bdrv_drain_all() instead but
test-thread-pool.c is lower level.

Since the test uses the global AioContext we can loop on qemu_aio_wait()
to wait for aio and bh activity to complete.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
