Fix potential deadlock when agent is closed

If the qemuAgentClose method is called from a place which holds
the domain lock, it is theoretically possible to get a deadlock
in the agent destroy callback. This has not been observed, but
the equivalent code in the QEMU monitor destroy callback has seen
a deadlock.

Remove the redundant locking while unrefing the object and the
bogus assignment
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>
(cherry picked from commit 362d0477)

src/qemu/qemu_process.c

@@ -129,7 +129,8 @@ qemuProcessHandleAgentEOF(qemuAgentPtr agent,
     virDomainObjLock(vm);
 
     priv = vm->privateData;
-    priv->agent = NULL;
+    if (priv->agent == agent)
+        priv->agent = NULL;
 
     virDomainObjUnlock(vm);
     qemuDriverUnlock(driver);
@@ -167,16 +168,9 @@ qemuProcessHandleAgentError(qemuAgentPtr agent ATTRIBUTE_UNUSED,
 static void qemuProcessHandleAgentDestroy(qemuAgentPtr agent,
                                           virDomainObjPtr vm)
 {
-    qemuDomainObjPrivatePtr priv;
-
     VIR_DEBUG("Received destroy agent=%p vm=%p", agent, vm);
 
-    virDomainObjLock(vm);
-    priv = vm->privateData;
-    if (priv->agent == agent)
-        priv->agent = NULL;
-    if (virObjectUnref(vm))
-        virDomainObjUnlock(vm);
+    virObjectUnref(vm);
 }