x86/HVMlite: document the BSP/AP boot ABI

x86/HVMlite: document the BSP/AP boot ABI

The discussion in [1] lead to an agreement of the missing pieces in PVH
(or HVM without a device-model) in order to progress with it's
implementation.

One of the missing pieces is a new boot ABI, that replaces the PV boot
ABI. The aim of this new boot ABI is to remove the limitations of the
PV boot ABI, that are no longer present when using auto-translated
guests. The new boot protocol should allow to use the same entry point
for both 32bit and 64bit guests, and let the guest choose it's bitness
and paging mode at run time without the domain builder knowing in
advance.

This patch introduces a new document called hvmlite.markdown, with the
intention of merging it into pvh.markdown once the HVMlite implementation
has feature parity with PVH and the old PVH ABI is replaced with the
HVMlite one.

[1] http://lists.xen.org/archives/html/xen-devel/2015-06/msg00258.html

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>