flush pending aio requests

When we finish migration, there may be pending async io requests
in flight. If we don't flush it before stage3 starting, it might be
the case that the guest loses it.

Signed-off-by: Glauber Costa <glommer@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

migration.c

diff --git a/migration.c b/migration.c
index e6c8b16..ee64d41 100644
--- a/migration.c
+++ b/migration.c
@@ -266,6 +266,7 @@
         dprintf("done iterating\n");
         vm_stop(0);
 
+        qemu_aio_flush();
         bdrv_flush_all();
         if ((qemu_savevm_state_complete(s->file)) < 0) {
             if (old_vm_running) {