xen: pci: collapse apic_register_gsi_xen_hvm and xen_hvm_register_pirq

apic_register_gsi_xen_hvm is a tiny wrapper around
xen_hvm_register_pirq.

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