xen: introduce xenbus_read_unsigned()

There are multiple instances of code reading an optional unsigned
parameter from Xenstore via xenbus_scanf(). Instead of repeating the
same code over and over add a service function doing the job.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>