xen: implement hook to fetch and parse e820 memory map

e820 memory map is fetched using a hypercall under Xen PVH, so add a
hook to init_ops in oder to diverge from bare metal and implement a
Xen variant.

Approved by: gibbs
Sponsored by: Citrix Systems R&D

x86/include/init.h:
 - Add a parse_memmap hook to init_ops, that will be called to fetch
   and parse the memory map.

amd64/amd64/machdep.c:
 - Decouple the fetch and the parse of the memmap, so the parse
   function can be shared with Xen code.
 - Move code around in order to implement the parse_memmap hook.

amd64/include/pc/bios.h:
 - Declare bios_add_smap_entries (implemented in machdep.c).

x86/xen/pv.c:
 - Implement fetching of e820 memmap when running as a PVH guest by
   using the XENMEM_memory_map hypercall.
xen: use the same hypercall mechanism for XEN and XENHVM

Currently XEN (PV) and XENHVM (PVHVM) ports use different ways to
issue hypercalls, unify this by filling the hypercall_page under HVM
also.

Approved by: gibbs
Sponsored by: Citrix Systems R&D

amd64/include/xen/hypercall.h:
 - Unify Xen hypercall code by always using the PV way.

i386/i386/locore.s:
 - Define hypercall_page on i386 XENHVM.

x86/xen/hvm.c:
 - Fill hypercall_page on XENHVM kernels using the HVM method (only
   when running as an HVM guest).
xen: add an apic_enumerator for PVH

On PVH there's no ACPI, so the CPU enumeration must be implemented
using Xen hypercalls.

Approved by: gibbs
Sponsored by: Citrix Systems R&D

x86/xen/pvcpu_enum.c:
 - Enumerate avaiable vCPUs on PVH by using the VCPUOP_is_up
   hypercall.
 - Set vcpu_id for PVH guests.

conf/files.amd64:
 - Include the PV CPU enumerator in the XENHVM build.
xen: add hook for AP bootstrap memory reservation

This hook will only be implemented for bare metal, Xen doesn't require
any bootstrap code since APs are started in long mode with paging
enabled.

Approved by: gibbs
Sponsored by: Citrix Systems R&D

amd64/amd64/machdep.c:
 - Set mp_bootaddress hook for bare metal.

x86/include/init.h:
 - Define mp_bootaddress in init_ops.
xen: changes to hvm code in order to support PVH guests

On PVH we don't need to init the shared info page, or disable emulated
devices. Also, make sure PV IPIs are set before starting the APs.

Approved by: gibbs
Sponsored by: Citrix Systems R&D

x86/xen/hvm.c:
 - Return early from functions that are no-ops on Xen PVH guests.
 - In order to make sure PV IPIs are setup before AP startup,
   initialize them in SI_SUB_SMP-1.
xen: add a hook to perform AP startup

AP startup on PVH follows the PV method, so we need to add a hook in
order to diverge from bare metal.

Approved by: gibbs
Sponsored by: Citrix Systems R&D

amd64/amd64/machdep.c:
 - Add hook for start_all_aps on native (using native_start_all_aps
   defined in mp_machdep).

amd64/amd64/mp_machdep.c:
 - Make some variables global because they will also be used by the
   Xen PVH AP startup code.
 - Use the start_all_aps hook to start APs.
 - Rename start_all_aps to native_start_all_aps.

amd64/include/smp.h:
 - Add declaration for native_start_all_aps.

x86/include/init.h:
 - Declare start_all_aps hook in init_ops.

x86/xen/pv.c:
 - Pick external declarations from mp_machdep.
 - Introduce Xen PV code to start APs on PVH.
 - Set start_all_aps init hook to use the Xen PVH implementation.