libvirt: avoid dead store in virDomainMigrateVersion3

* src/qemu/qemu_migration.c: avoid dead 'ret' assignment and silence
  clang warning.

Detected by ccc-analyzer:

libvirt.c:4277:5: warning: Value stored to 'ret' is never read
    ret = domain->conn->driver->domainMigrateConfirm3
    ^     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

src/libvirt.c

@@ -4289,6 +4289,9 @@ confirm:
      * do, but fortunately worst case is that there is a
      * domain left in 'paused' state on source.
      */
+    if (ret < 0)
+        VIR_WARN("Guest %s probably left in 'paused' state on source",
+                 domain->name);
 
  done:
     if (orig_err) {