migration: block-dirty-bitmap: add missing qemu_mutex_lock_iothread

init_dirty_bitmap_migration assumes the iothread lock (BQL)
to be held, but instead it isn't.

Instead of adding the lock to qemu_savevm_state_setup(),
follow the same pattern as the other ->save_setup callbacks
and lock+unlock inside dirty_bitmap_save_setup().

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20211005080751.3797161-2-eesposit@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

migration/block-dirty-bitmap.c

@@ -1215,7 +1215,10 @@ static int dirty_bitmap_save_setup(QEMUFile *f, void *opaque)
 {
     DBMSaveState *s = &((DBMState *)opaque)->save;
     SaveBitmapState *dbms = NULL;
+
+    qemu_mutex_lock_iothread();
     if (init_dirty_bitmap_migration(s) < 0) {
+        qemu_mutex_unlock_iothread();
         return -1;
     }
 
@@ -1223,7 +1226,7 @@ static int dirty_bitmap_save_setup(QEMUFile *f, void *opaque)
         send_bitmap_start(f, s, dbms);
     }
     qemu_put_bitmap_flags(f, DIRTY_BITMAP_MIG_FLAG_EOS);
-
+    qemu_mutex_unlock_iothread();
     return 0;
 }