qemu: Do not fail virConnectCompareCPU if host CPU is not known

When host CPU could not be properly detected, virConnectCompareCPU will
just report that any CPU is incompatible with host CPU instead of
failing.
(cherry picked from commit 87c86231)

src/qemu/qemu_driver.c

@@ -9322,9 +9322,12 @@ qemuCPUCompare(virConnectPtr conn,
 
     qemuDriverLock(driver);
 
-    if (!driver->caps || !driver->caps->host.cpu) {
-        qemuReportError(VIR_ERR_OPERATION_INVALID,
-                        "%s", _("cannot get host CPU capabilities"));
+    if (!driver->caps) {
+        qemuReportError(VIR_ERR_INTERNAL_ERROR,
+                        "%s", _("cannot get host capabilities"));
+    } else if (!driver->caps->host.cpu) {
+        VIR_WARN("cannot get host CPU capabilities");
+        ret = VIR_CPU_COMPARE_INCOMPATIBLE;
     } else {
         ret = cpuCompareXML(driver->caps->host.cpu, xmlDesc);
     }