docs: libxc migration stream specification

docs: libxc migration stream specification

Add the specification for a new migration stream format.  The document
includes all the details but to summarize:

The existing (legacy) format is dependant on the word size of the
toolstack.  This prevents domains from migrating from hosts running
32-bit toolstacks to hosts running 64-bit toolstacks (and vice-versa).

The legacy format lacks any version information making it difficult to
extend in compatible way.

The new format has a header (the image header) with version information,
a domain header with basic information of the domain and a stream of
records for the image data.

The format will be used for future domain types (such as on ARM).

The specification is pandoc format (an extended markdown format) and the
documentation build system is extended to support pandoc format documents.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>