qemu: driver: Validate configuration when setting maximum vcpu count

Setting vcpu count when cpu topology is specified may result into an
invalid configuration. Since the topology can't be modified, reject the
setting if it doesn't match the requested topology. This will allow
fixing the topology in case it was broken.

Partially fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1370066

src/qemu/qemu_driver.c

@@ -4730,6 +4730,18 @@ qemuDomainSetVcpusMax(virQEMUDriverPtr driver,
         goto cleanup;
     }
 
+    if (persistentDef && persistentDef->cpu && persistentDef->cpu->sockets) {
+        /* allow setting a valid vcpu count for the topology so an invalid
+         * setting may be corrected via this API */
+        if (nvcpus != persistentDef->cpu->sockets *
+                      persistentDef->cpu->cores *
+                      persistentDef->cpu->threads) {
+            virReportError(VIR_ERR_INVALID_ARG, "%s",
+                           _("CPU topology doesn't match the desired vcpu count"));
+            goto cleanup;
+        }
+    }
+
     if (virDomainDefSetVcpusMax(persistentDef, nvcpus, driver->xmlopt) < 0)
         goto cleanup;