Format printf format specifier used with niothreadids

The niothreadids struct field is size_t, so must use %zu not %lu
with printf. While they're identical on some platforms, on others
they are different, causing warnings

conf/domain_conf.c: In function 'virDomainDefCheckABIStabilityFlags':
conf/domain_conf.c:19575:26: error: format '%lu' expects argument of type 'long unsigned int', but argument 7 has type 'size_t {aka unsigned int}' [-Werror=format=]
                        _("Target domain iothreads count %lu does not "
                          ^

conf/domain_conf.c: In function 'virDomainDefFormatInternal':
conf/domain_conf.c:23915:46: error: format '%lu' expects argument of type 'long unsigned int', but argument 3 has type 'size_t {aka unsigned int}' [-Werror=format=]
         virBufferAsprintf(buf, "<iothreads>%lu</iothreads>\n",
                                              ^
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

src/conf/domain_conf.c

@@ -19572,8 +19572,8 @@ virDomainDefCheckABIStabilityFlags(virDomainDefPtr src,
 
     if (src->niothreadids != dst->niothreadids) {
         virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
-                       _("Target domain iothreads count %lu does not "
-                         "match source %lu"),
+                       _("Target domain iothreads count %zu does not "
+                         "match source %zu"),
                        dst->niothreadids, src->niothreadids);
         goto error;
     }
@@ -23912,7 +23912,7 @@ virDomainDefFormatInternal(virDomainDefPtr def,
         goto error;
 
     if (def->niothreadids > 0) {
-        virBufferAsprintf(buf, "<iothreads>%lu</iothreads>\n",
+        virBufferAsprintf(buf, "<iothreads>%zu</iothreads>\n",
                           def->niothreadids);
         if (virDomainDefIothreadShouldFormat(def)) {
             virBufferAddLit(buf, "<iothreadids>\n");