Error out on missing machine type in machine configs

Commit f1a89a8 allowed parsing configs from /etc/libvirt
without validating the emulator capabilities.

Check for the presence of os->type.machine even if the
VIR_DOMAIN_DEF_PARSE_SKIP_OSTYPE_CHECKS flag is set,
otherwise the daemon can crash on carelessly crafted input
in the config directory.

https://bugzilla.redhat.com/show_bug.cgi?id=1267256

src/conf/domain_conf.c

@@ -14841,6 +14841,12 @@ virDomainDefParseXML(xmlDocPtr xml,
             goto error;
         }
         VIR_FREE(capsdata);
+    } else {
+        if (!def->os.machine) {
+            virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
+                           _("Missing machine type"));
+            goto error;
+        }
     }
 
     /* Extract domain name */