libxl: fix ARM build after 54ed251dc7

Commit "tools: Rework xc_domain_create() to take a full
xen_domctl_createdomain"  failed to replace one further instance of
xc_config in libxl__arch_domain_save_config().

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/libxl/libxl_arm.c

@@ -125,7 +125,7 @@ int libxl__arch_domain_save_config(libxl__gc *gc,
         return ERROR_FAIL;
     }
 
-    state->clock_frequency = xc_config->clock_frequency;
+    state->clock_frequency = config->arch.clock_frequency;
 
     return 0;
 }