qemu: attach: Close monitor socket on connection failure

If attaching to a qemu process fails after opening the monitor socket
libvirt does not clean up the monitor. As the monitor also holds a
reference to the domain object the qemu attach API basically leaks it.

QEMU also does not interact on a second monitor connection and thus a
further attempt to attach to it would lock up.

Prevent libvirt from leaking the monitor by explicitly closing it.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1378401

src/qemu/qemu_process.c

@@ -6386,6 +6386,9 @@ int qemuProcessAttach(virConnectPtr conn ATTRIBUTE_UNUSED,
     if (active && virAtomicIntDecAndTest(&driver->nactive) &&
         driver->inhibitCallback)
         driver->inhibitCallback(false, driver->inhibitOpaque);
+
+    qemuMonitorClose(priv->mon);
+    priv->mon = NULL;
     qemuDomainLogContextFree(logCtxt);
     VIR_FREE(seclabel);
     VIR_FREE(sec_managers);