xen/pci: Provide #ifdef CONFIG_ACPI to easy code squashing.

In the past we would guard those code segments to be dependent
on CONFIG_XEN_DOM0 (which depends on CONFIG_ACPI) so this patch is
not stricly necessary. But the next patch will merge common
HVM and initial domain code and we want to make sure the CONFIG_ACPI
dependency is preserved - as HVM code does not depend on CONFIG_XEN_DOM0.

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