conf: disallow empty cpusets for vcpu pinning when parsing XML

They are disallowed in the pinning API and as default cpuset.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1293241

src/conf/domain_conf.c

@@ -14163,6 +14163,12 @@ virDomainVcpuPinDefParseXML(xmlNodePtr node,
     if (virBitmapParse(tmp, 0, &def->cpumask, VIR_DOMAIN_CPUMASK_LEN) < 0)
         goto error;
 
+    if (virBitmapIsAllClear(def->cpumask)) {
+        virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
+                       _("Invalid value of 'cpuset': %s"), tmp);
+        goto error;
+    }
+
  cleanup:
     VIR_FREE(tmp);
     ctxt->node = oldnode;