xen: do not compile manage, balloon, pci, acpi, pcpu and cpu_hotplug on ARM

Changes in v4:
- compile pcpu only on x86;
- use "+=" instead of ":=" for dom0- targets.

Changes in v2:

- make pci.o depend on CONFIG_PCI and acpi.o depend on CONFIG_ACPI.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>