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

See git commit f10cd522
(xen: disable PV spinlocks on HVM) for details.

But we did not disable it everywhere - which means that when
we boot as PVHVM we end up allocating per-CPU irq line for
spinlock. This fixes that.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>