xen: support runqueue steal time on xen

Up to now reading the stolen time of a remote cpu was not possible in a
performant way under Xen. This made support of runqueue steal time via
paravirt_steal_rq_enabled impossible.

With the addition of an appropriate hypervisor interface this is now
possible, so add the support.

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