qemu: Allow cachetune only for KVM domains

https://bugzilla.redhat.com/show_bug.cgi?id=1541921

In TCG mode, there are no vCPU threads and thus there's nothing
to be placed into resctrl group. Forbid such configuration.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

src/qemu/qemu_domain.c

@@ -3986,6 +3986,13 @@ qemuDomainDefValidate(const virDomainDef *def,
         }
     }
 
+    if (def->ncachetunes &&
+        def->virtType != VIR_DOMAIN_VIRT_KVM) {
+        virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
+                       _("cachetune is only supported for KVM domains"));
+        goto cleanup;
+    }
+
     if (qemuDomainDefValidateFeatures(def, qemuCaps) < 0)
         goto cleanup;