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.