screenshot: Set access rights to temporary file

Although we create a temporary file, it is owned by root:root and have
rights 0600. In case qemu does not run under root, it is unable to write
to that file and thus we transfer 0B sized file.

src/qemu/qemu_driver.c

@@ -2729,6 +2729,8 @@ qemuDomainScreenshot(virDomainPtr dom,
         goto endjob;
     }
 
+    virSecurityManagerSetSavedStateLabel(qemu_driver->securityManager, vm, tmp);
+
     qemuDomainObjEnterMonitor(vm);
     if (qemuMonitorScreendump(priv->mon, tmp) < 0) {
         qemuDomainObjExitMonitor(vm);