block: convert qemu_aio_flush() calls to bdrv_drain_all()

Many places in QEMU call qemu_aio_flush() to complete all pending
asynchronous I/O.  Most of these places actually want to drain all block
requests but there is no block layer API to do so.

This patch introduces the bdrv_drain_all() API to wait for requests
across all BlockDriverStates to complete.  As a bonus we perform checks
after qemu_aio_wait() to ensure that requests really have finished.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
