xen: pci: only define xen_initdom_setup_msi_irqs if CONFIG_XEN_DOM0

Fixes:
 CC      arch/x86/pci/xen.o
arch/x86/pci/xen.c:183: warning: 'xen_initdom_setup_msi_irqs' defined but not used

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