Handle BH's queued by AIO completions in qemu_aio_flush()

Without this, the call to qemu_aio_flush during migration doesn't
actually flush all in-flight SCSI IOs.

Signed-off-by: Nolan Leake <nolan <at> sigbus.net>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
