docs/migration Specify migration v3 and STATIC_DATA_END

docs/migration Specify migration v3 and STATIC_DATA_END

Migration data can be split into two parts - that which is invariant of
guest execution, and that which is not.  Separate these two with the
STATIC_DATA_END record.

The short term, we want to move the x86 CPU Policy data into the stream.
In the longer term, we want to provisionally send the static data only
to the destination as a more robust compatibility check.  In both cases,
we will want a callback into the higher level toolstack.

Mandate the presence of the STATIC_DATA_END record, and declare this v3,
along with instructions for how to compatibly interpret a v2 stream.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>