conf: Provide error on undefined vcpusched entry

Modify virDomainDefGetVcpuSched to emit an error message if
virDomainDefGetVcpu returns NULL meaning the vcpu could not
be found. Prior to commit id '9cc931f0b' the error message
would have been issued in virDomainDefGetVcpu.

src/conf/domain_conf.c

@@ -1480,8 +1480,12 @@ virDomainDefGetVcpuSched(virDomainDefPtr def,
 {
     virDomainVcpuDefPtr vcpuinfo;
 
-    if (!(vcpuinfo = virDomainDefGetVcpu(def, vcpu)))
+    if (!(vcpuinfo = virDomainDefGetVcpu(def, vcpu))) {
+        virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
+                       _("vCPU '%u' is not present in domain definition"),
+                       vcpu);
         return NULL;
+    }
 
     return &vcpuinfo->sched;
 }