xen: don't bother to stop other cpus on shutdown/reboot

Xen will shoot all the VCPUs when we do a shutdown hypercall, so there's
no need to do it manually.

In any case it will fail because all the IPI irqs have been pulled
down by this point, so the cross-CPU calls will simply hang forever.

Until change 76fac077 the function calls
were not synchronously waited for, so this wasn't apparent.  However after
that change the calls became synchronous leading to a hang on shutdown
on multi-VCPU guests.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Stable Kernel <stable@kernel.org>
Cc: Alok Kataria <akataria@vmware.com>