xen/smp: Move the common CPU init code a bit to prep for PVH patch.

The PV and PVH code CPU init code share some functionality. The
PVH code ("xen/pvh: Extend vcpu_guest_context, p2m, event, and XenBus")
sets some of these up, but not all. To make it easier to read, this
patch removes the PV specific out of the generic way.

No functional change - just code movement.

Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
[v2: Fixed compile errors noticed by Fengguang Wu build system]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>