CP-4042: Add VM.generation-id to API

We are adding the API glue necessary to use the new GenID feature that the Windows team have implemented. See the first CP-4042 commit message for a description of the work.

This pull request also makes some minor fixes to xenguest_stubs.c and Xapi.random_setup, and bumps the datamodel schema number.

Signed-off-by: Mike McClurg <mike.mcclurg@citrix.com>