tools/libxl: document libxl memory management policy

tools/libxl: document libxl memory management policy

After animated discussion with several libxl developers we seem to have
agreed on a policy for memory management within libxenlight. These
comments document the policy which is mostly implemented since
21977:51147d5b17c3 but some aspects (comments, function naming) are
guidelines to be followed in future functionality and perhaps to be
implemented by search/replace in future patches.

The document is mostly authored by Ian Jackson but with modifications to
reflect the slightly different functionality that has been implemented
since this was proposed.

Signed-off-by: Gianni Tedesco <gianni.tedesco@citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>