qemu: Fix crash when defining XML with bogus emulator

We weren't checking for failure of qemuCaps lookup

src/qemu/qemu_domain.c

@@ -1247,7 +1247,9 @@ qemuDomainDefPostParse(virDomainDefPtr def,
         !(def->emulator = virDomainDefGetDefaultEmulator(def, caps)))
         return ret;
 
-    qemuCaps = virQEMUCapsCacheLookup(driver->qemuCapsCache, def->emulator);
+    if (!(qemuCaps = virQEMUCapsCacheLookup(driver->qemuCapsCache,
+                                            def->emulator)))
+        goto cleanup;
 
     if (qemuDomainDefAddDefaultDevices(def, qemuCaps) < 0)
         goto cleanup;