Fix parsing of arch from domain XML

When parsing the arch from domain XML, the result was only
saved to a local variable, not the virDomainDefPtr
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

src/conf/domain_conf.c

@@ -9576,8 +9576,8 @@ static virDomainDefPtr virDomainDefParseXML(virCapsPtr caps,
 
     tmp = virXPathString("string(./os/type[1]/@arch)", ctxt);
     if (tmp) {
-        virArch arch = virArchFromString(tmp);
-        if (!arch) {
+        def->os.arch = virArchFromString(tmp);
+        if (!def->os.arch) {
             virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
                            _("Unknown architecture %s"),
                            tmp);