libvirt: Add virStorageVolDeleteFlags to virStorageVolDelete

Although they've been present for quite a while, they weren't added
to the API definition, so add them there to make it clearer.

Currently only the RBD backend even checks for any flags.

src/libvirt-storage.c

@@ -1684,7 +1684,7 @@ virStorageVolUpload(virStorageVolPtr vol,
 /**
  * virStorageVolDelete:
  * @vol: pointer to storage volume
- * @flags: extra flags; not used yet, so callers should always pass 0
+ * @flags: bitwise-OR of virStorageVolDeleteFlags
  *
  * Delete the storage volume from the pool
  *