xen/dom0less: Clear xenstore page

C Xenstored was seen failing to map a domU's event channel.  Enhancing
Xen's error messages shows:
common/event_channel.c:427:d0v0 EVTCHNOP failure: d1, port 3267543746 error -22

Port 3267543746 is 0xc2c2c2c2 - SCRUB_PATTERN.

Xenstore expects a non-zero evtchn_port to be valid.  Explicitly zero
the page to avoid this.

Fixes: df2c03e2eb ("xen/arm: Alloc XenStore page for Dom0less DomUs from hypervisor")
Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Acked-by: Michal Orzel <michal.orzel@amd.com>

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

@@ -600,6 +600,7 @@ static int __init alloc_xenstore_page(struct domain *d)
     d->arch.hvm.params[HVM_PARAM_STORE_PFN] = gfn_x(gfn);
 #endif
     interface = map_domain_page(mfn);
+    clear_page(interface);
     interface->connection = XENSTORE_RECONNECT;
     unmap_domain_page(interface);