lock qemu_driver early in qemuGetSchedulerParametersFlags()

If we pass VIR_DOMAIN_AFFECT_LIVE | VIR_DOMAIN_AFFECT_CONFIG to
qemuGetSchedulerParametersFlags() or *nparams is less than 1,
we will unlock qemu_driver without locking it. It's very dangerous.

We should lock qemu_driver after calling virCheckFlags().

src/qemu/qemu_driver.c

@@ -5837,6 +5837,8 @@ qemuGetSchedulerParametersFlags(virDomainPtr dom,
     virCheckFlags(VIR_DOMAIN_AFFECT_LIVE |
                   VIR_DOMAIN_AFFECT_CONFIG, -1);
 
+    qemuDriverLock(driver);
+
     if ((flags & (VIR_DOMAIN_AFFECT_LIVE | VIR_DOMAIN_AFFECT_CONFIG)) ==
         (VIR_DOMAIN_AFFECT_LIVE | VIR_DOMAIN_AFFECT_CONFIG)) {
         qemuReportError(VIR_ERR_INVALID_ARG, "%s",
@@ -5850,7 +5852,6 @@ qemuGetSchedulerParametersFlags(virDomainPtr dom,
         goto cleanup;
     }
 
-    qemuDriverLock(driver);
     vm = virDomainFindByUUID(&driver->domains, dom->uuid);
 
     if (vm == NULL) {