xen/irq: Cleanup up the pirq_to_irq for DomU PV PCI passthrough guests as well.

We only did this for PV guests that are xen_initial_domain() but
there is not reason not to do this for other cases. The other
case is only exercised when you pass in a PCI device to a PV guest
_and_ the device in question.

Reviewed-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>