qemu: Don't update count of vCPUs if hot-plug fails silently

When cpu hotplug fails without reporting an error, we would fail the
command but update the count of vCPUs anyways.

Commit 761fc48136 fixed the case when CPU
hot-unplug failed silently, but forgot to fix up the value in this case.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1000357

src/qemu/qemu_driver.c

@@ -3982,6 +3982,7 @@ static int qemuDomainHotplugVcpus(virQEMUDriverPtr driver,
                        _("got wrong number of vCPU pids from QEMU monitor. "
                          "got %d, wanted %d"),
                        ncpupids, vcpus);
+        vcpus = oldvcpus;
         ret = -1;
         goto cleanup;
     }