qemu: perf: Fix crash/memory corruption on failed VM start

The new perf code didn't bother to clear a pointer in 'priv' causing a
double free or other memory corruption goodness if a VM failed to start.

Clear the pointer after freeing the memory.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1324757

src/qemu/qemu_process.c

@@ -3444,6 +3444,7 @@ qemuDomainPerfRestart(virDomainObjPtr vm)
 
  cleanup:
     virPerfFree(priv->perf);
+    priv->perf = NULL;
     return -1;
 }
 
@@ -5970,6 +5971,7 @@ void qemuProcessStop(virQEMUDriverPtr driver,
     virCgroupFree(&priv->cgroup);
 
     virPerfFree(priv->perf);
+    priv->perf = NULL;
 
     qemuProcessRemoveDomainStatus(driver, vm);