The definition of when qemu_aio_flush should loop is much simpler
than it looks.  It just has to call qemu_aio_wait until it makes
no progress and all flush callbacks return false.  qemu_aio_wait
is the logical place to tell the caller about this.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>