xen/smp/pvhvm: Don't initialize IRQ_WORKER as we are using the native one.

There is no need to use the PV version of the IRQ_WORKER mechanism
as under PVHVM we are using the native version. The native
version is using the SMP API.

They just sit around unused:

  69:          0          0  xen-percpu-ipi       irqwork0
  83:          0          0  xen-percpu-ipi       irqwork1

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