migration: hold AioContext lock for loadvm qemu_fclose()

migration_incoming_state_destroy() uses qemu_fclose() on the vmstate
file.  Make sure to call it inside an AioContext acquire/release region.

This fixes an 'qemu: qemu_mutex_unlock: Operation not permitted' abort
in loadvm.

This patch closes the vmstate file before ending the drained region.
Previously we closed the vmstate file after ending the drained region.
The order does not matter.

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

migration/savevm.c

@@ -2300,11 +2300,11 @@ int load_snapshot(const char *name, Error **errp)
 
     aio_context_acquire(aio_context);
     ret = qemu_loadvm_state(f);
+    migration_incoming_state_destroy();
     aio_context_release(aio_context);
 
     bdrv_drain_all_end();
 
-    migration_incoming_state_destroy();
     if (ret < 0) {
         error_setg(errp, "Error %d while loading VM state", ret);
         return ret;