qemu: Fix coldplug of vcpus

virDomainDefSetVcpus was not designed to handle coldplug of vcpus now
that we can set state of vcpus individually.

Introduce qemuDomainSetVcpusConfig that properly handles state changes
of vcpus when coldplugging so that invalid configurations are not
created.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1375939

src/qemu/qemu_driver.c

@@ -4901,6 +4901,58 @@ qemuDomainSetVcpusLive(virQEMUDriverPtr driver,
 }
 
 
+/**
+ * qemuDomainSetVcpusConfig:
+ * @def: config/offline definition of a domain
+ * @nvcpus: target vcpu count
+ *
+ * Properly handle cold(un)plug of vcpus:
+ * - plug in inactive vcpus/uplug active rather than rewriting state
+ * - fix hotpluggable state
+ */
+static void
+qemuDomainSetVcpusConfig(virDomainDefPtr def,
+                         unsigned int nvcpus)
+{
+    virDomainVcpuDefPtr vcpu;
+    size_t curvcpus = virDomainDefGetVcpus(def);
+    size_t maxvcpus = virDomainDefGetVcpusMax(def);
+    size_t i;
+
+
+    if (curvcpus == nvcpus)
+        return;
+
+    if (curvcpus < nvcpus) {
+        for (i = 0; i < maxvcpus; i++) {
+            vcpu = virDomainDefGetVcpu(def, i);
+
+            if (!vcpu || vcpu->online)
+                continue;
+
+            vcpu->online = true;
+            vcpu->hotpluggable = VIR_TRISTATE_BOOL_NO;
+
+            if (++curvcpus == nvcpus)
+                break;
+        }
+    } else {
+        for (i = maxvcpus; i != 0; i--) {
+            vcpu = virDomainDefGetVcpu(def, i - 1);
+
+            if (!vcpu || !vcpu->online)
+                continue;
+
+            vcpu->online = false;
+            vcpu->hotpluggable = VIR_TRISTATE_BOOL_YES;
+
+            if (--curvcpus == nvcpus)
+                break;
+        }
+    }
+}
+
+
 static int
 qemuDomainSetVcpusInternal(virQEMUDriverPtr driver,
                            virDomainObjPtr vm,
@@ -4931,8 +4983,7 @@ qemuDomainSetVcpusInternal(virQEMUDriverPtr driver,
         goto cleanup;
 
     if (persistentDef) {
-        if (virDomainDefSetVcpus(persistentDef, nvcpus) < 0)
+        qemuDomainSetVcpusConfig(persistentDef, nvcpus);
-            goto cleanup;
 
         if (virDomainSaveConfig(cfg->configDir, driver->caps, persistentDef) < 0)
             goto cleanup;