util: storage: Fix check for empty storage device

If the storage device type is parsed as network our parser still allows
it to omit the <source> element. The empty drive check would not trigger
on such device as it expects that every network storage source is valid.

Use VIR_STORAGE_NET_PROTOCOL_NONE as a marker that the storage source is
empty.

src/util/virstoragefile.c

@@ -1989,6 +1989,10 @@ virStorageSourceIsEmpty(virStorageSourcePtr src)
     if (src->type == VIR_STORAGE_TYPE_NONE)
         return true;
 
+    if (src->type == VIR_STORAGE_TYPE_NETWORK &&
+        src->protocol == VIR_STORAGE_NET_PROTOCOL_NONE)
+        return true;
+
     return false;
 }