Revert "xen/spinlock: Disable IRQ spinlock (PV) allocation on PVHVM"

This reverts commit 70dd4998.

Now that the bugs have been resolved we can re-enable the
PV ticketlock implementation under PVHVM Xen guests.

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