qemu: Set fake reboot flag to false when mode=agent

As commit id 'e2b86f580' notes, when mode=agent possibly setting the
fake reboot flag to true wouldn't be necessary; however, it doesn't
"force" the issue by just ensuring the fake reboot is false, so this
patch adds the explicit setting for the reboot path.

More investigation and details can be found in commit id '8be502fd'
as well as in the archives at:

  https://www.redhat.com/archives/libvir-list/2015-April/msg00715.html

src/qemu/qemu_driver.c

@@ -2092,6 +2092,7 @@ qemuDomainReboot(virDomainPtr dom, unsigned int flags)
     }
 
     if (useAgent) {
+        qemuDomainSetFakeReboot(driver, vm, false);
         qemuDomainObjEnterAgent(vm);
         ret = qemuAgentShutdown(priv->agent, agentFlag);
         qemuDomainObjExitAgent(vm);