qemu: Minimalize global driver accesses

Whilst working on another issue, I've noticed that in some
functions we have a local @driver variable among with access to
global @qemu_driver variable. This makes no sense.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

src/qemu/qemu_driver.c

@@ -3835,7 +3835,7 @@ qemuDomainScreenshot(virDomainPtr dom,
     }
     unlink_tmp = true;
 
-    virSecurityManagerSetSavedStateLabel(qemu_driver->securityManager, vm->def, tmp);
+    virSecurityManagerSetSavedStateLabel(driver->securityManager, vm->def, tmp);
 
     qemuDomainObjEnterMonitor(driver, vm);
     if (qemuMonitorScreendump(priv->mon, tmp) < 0) {
@@ -11460,7 +11460,7 @@ qemuDomainMemoryPeek(virDomainPtr dom,
         goto endjob;
     }
 
-    virSecurityManagerSetSavedStateLabel(qemu_driver->securityManager, vm->def, tmp);
+    virSecurityManagerSetSavedStateLabel(driver->securityManager, vm->def, tmp);
 
     priv = vm->privateData;
     qemuDomainObjEnterMonitor(driver, vm);