qemu: avoid dead store in doPeer2PeerMigrate3

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

Detected by ccc-analyzer:

  CC     libvirt_driver_qemu_la-qemu_migration.lo
qemu/qemu_migration.c:2046:5: warning: Value stored to 'ret' is never read
    ret = qemuMigrationConfirm(driver, sconn, vm,
    ^     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

src/qemu/qemu_migration.c

@@ -2043,6 +2043,9 @@ finish:
      * 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",
+                 vm->def->name);
 
  cleanup:
     if (ddomain) {