tools/xenstored: Read event channel from xenstored page

Make introduce_domain() use an event channel from the the xenstore page.
It is only used if non-zero.  Otherwise the passed in event channel port
is used.

The is useful for a xenstored stubdom to configure domains autonomously.

Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>
Reviewed-by: Juergen Gross <jgross@suse.com>

tools/xenstored/domain.c

@@ -1036,6 +1036,10 @@ static struct domain *introduce_domain(const void *ctx,
 		interface = map_interface(domid);
 		if (!interface && !restore)
 			return NULL;
+
+		if (interface->evtchn_port)
+			port = interface->evtchn_port;
+
 		if (new_domain(domain, port, restore)) {
 			rc = errno;
 			if (interface)