qemu: Invert condition nesting in qemuDomainDefValidate()

While at the moment we're only performing a single check that is
connected to vCPU hotplugging, we're going to introduce a second
one soon. Move the topology check underneath the capability check
to make that easier; since, after this change, the 'topologycpus'
variable doesn't need to have function scope, we move its
declaration to the inner scope as well.

The comments around the check are modified in order to explain
the different QEMU versions involved.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>

src/qemu/qemu_domain.c

@@ -3299,7 +3299,6 @@ qemuDomainDefValidate(const virDomainDef *def,
 {
     virQEMUDriverPtr driver = opaque;
     virQEMUCapsPtr qemuCaps = NULL;
-    unsigned int topologycpus;
     int ret = -1;
 
     if (!(qemuCaps = virQEMUCapsCacheLookup(driver->qemuCapsCache,
@@ -3359,11 +3358,17 @@ qemuDomainDefValidate(const virDomainDef *def,
         }
     }
 
-    /* qemu as of 2.5.0 rejects SMP topologies that don't match the cpu count */
-    if (virDomainDefGetVcpusTopology(def, &topologycpus) == 0 &&
-        topologycpus != virDomainDefGetVcpusMax(def)) {
-        /* presence of query-hotpluggable-cpus should be a good enough witness */
-        if (virQEMUCapsGet(qemuCaps, QEMU_CAPS_QUERY_HOTPLUGGABLE_CPUS)) {
+    /* QEMU 2.7 (detected via the availability of query-hotpluggable-cpus)
+     * enforces stricter rules than previous versions when it comes to guest
+     * CPU topology. Verify known constraints are respected */
+    if (virQEMUCapsGet(qemuCaps, QEMU_CAPS_QUERY_HOTPLUGGABLE_CPUS)) {
+        unsigned int topologycpus;
+
+        /* Starting from QEMU 2.5, max vCPU count and overall vCPU topology
+         * must agree. We only actually enforce this with QEMU 2.7+, due
+         * to the capability check above */
+        if (virDomainDefGetVcpusTopology(def, &topologycpus) == 0 &&
+            topologycpus != virDomainDefGetVcpusMax(def)) {
             virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
                            _("CPU topology doesn't match maximum vcpu count"));
             goto cleanup;