xen/pvh: Extend vcpu_guest_context, p2m, event, and XenBus.

xen/pvh: Extend vcpu_guest_context, p2m, event, and XenBus.

Make gdt_frames[]/gdt_ents into a union with {gdtaddr, gdtsz},
as PVH only needs to send down gdtaddr and gdtsz in the
vcpu_guest_context structure..

For interrupts, PVH uses native_irq_ops so we can skip most of the
PV ones. In the future we can support the pirq_eoi_map..
Also VCPU hotplug is currently not available for PVH.

For events (and IRQs) we follow what PVHVM does - so use callback
vector.  Lastly, for XenBus we use the same logic that is used in
the PVHVM case.

Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Mukesh Rathor <mukesh.rathor@oracle.com>
[v2: Rebased it]
[v3: Move 64-bit ifdef and based on Stefan add extra comments.]
[v4: Rebased it once more]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>