dom0less: Move single-xenstore check from bindings to set_xs_domain()

Later patches extract the bindings to a separate file, where that global
won't be available. Make the check when calling the function instead so
the bindings remain agnostic.

Signed-off-by: Alejandro Vallejo <alejandro.garciavallejo@amd.com>
Reviewed-by: Jason Andryuk <jason.andryuk@amd.com>
Reviewed-by: Denis Mukhin <dmukhin@ford.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

xen/common/device-tree/dom0less-build.c

@@ -38,6 +38,9 @@ static bool __initdata need_xenstore;
 
 void __init set_xs_domain(struct domain *d)
 {
+    if ( xs_domid != DOMID_INVALID )
+        panic("Only 1 xenstore domain can be specified! (%u)", xs_domid);
+
     xs_domid = d->domain_id;
     set_global_virq_handler(d, VIRQ_DOM_EXC);
 }
@@ -876,10 +879,6 @@ void __init create_domUs(void)
 
             if ( val & DOMAIN_CAPS_XENSTORE )
             {
-                if ( xs_domid != DOMID_INVALID )
-                    panic("Only 1 xenstore domain can be specified! (%u)\n",
-                            xs_domid);
-
                 d_cfg.flags |= XEN_DOMCTL_CDF_xs_domain;
                 d_cfg.max_evtchn_port = -1;
             }