xenbus: frontend resume cleanup

Only create the delayed resume workqueue if we are running in the same domain
as xenstored and issue a warning if the workqueue creation fails.

Move the work initialization to the device probe so it is done only once.

Signed-off-by: Aurelien Chartier <aurelien.chartier@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>