docs: Introduce specification for x86 pv bootloader chainloading paths/formats.

docs: Introduce specification for x86 pv bootloader chainloading paths/formats.

In order to support pvgrub (or other bootloader) from dom0 chainloading a
pvgrub (or other) from within the domU filesystem we need a standard for where
the stage 1 bootloader should look and what it should expect to find there.

Add a document along those lines.

This is currently x86-specific since it is expected that other new architecures
(including ARM) will either not need to implement this due to the existence of
more generic specifications for that arch (e.g. the ARM VM spec) or will update
this document in due course.

Signed-off-by: Ian Campbell <ijc@hellion.org.uk>
Acked-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Cc: Colin Watson <cjwatson@debian.org>
Cc: 759018@bugs.debian.org