qemu: perf: Tweak flags before using them

@flags have a valid modification impact only after calling
virDomainObjUpdateModificationImpact. virDomainObjGetOneDef calls it but
doesn't update them in the caller.

src/qemu/qemu_driver.c

@@ -10150,6 +10150,9 @@ qemuDomainGetPerfEvents(virDomainPtr dom,
     if (qemuDomainObjBeginJob(driver, vm, QEMU_JOB_QUERY) < 0)
         goto cleanup;
 
+    if (virDomainObjUpdateModificationImpact(vm, &flags) < 0)
+        goto endjob;
+
     if (!(def = virDomainObjGetOneDef(vm, flags)))
         goto endjob;