xen: introduce XEN_DOM0 as a silent option

Add XEN_DOM0 to arch/x86/xen/Kconfig as a silent compile time option
that gets enabled when xen and basic x86, acpi and pci support are
selected.

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