qemu: use bdrv_flush_all for vm_stop et al

Reimplement bdrv_flush_all for vm_stop. In contrast to blk_flush_all,
bdrv_flush_all does not have device model restrictions. This allows
us to flush and halt unconditionally without error.

This allows us to do things like migrate when we have a device with
an open tray, but has a node that may need to be flushed, or nodes
that aren't currently attached to any device and need to be flushed.

Specifically, this allows us to migrate when we have a CDROM with
an open tray.

Signed-off-by: John Snow <[email protected]>
Reviewed-by: Kevin Wolf <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Acked-by: Fam Zheng <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>

cpus.c

@@ -751,7 +751,7 @@ static int do_vm_stop(RunState state)
 
     bdrv_drain_all();
     replay_disable_events();
-    ret = blk_flush_all();
+    ret = bdrv_flush_all();
 
     return ret;
 }
@@ -1408,7 +1408,7 @@ int vm_stop_force_state(RunState state)
         bdrv_drain_all();
         /* Make sure to return an error if the flush in a previous vm_stop()
          * failed. */
-        return blk_flush_all();
+        return bdrv_flush_all();
     }
 }