fix qemu_aio_flush

fix qemu_aio_flush

qemu_aio_wait by invoking the bh or one of the aio completion
callbacks, could end up submitting new pending aio, breaking the
invariant that qemu_aio_flush returns only when no pending aio is
outstanding (possibly a problem for migration as such).

Signed-off-by: Andrea Arcangeli <[email protected]>
Signed-off-by: Christoph Hellwig <[email protected]>
Acked-by: Kevin Wolf <[email protected]>