Add a minimum reservation feature to the pool code and use it in GNTTAB

The grant table reservation is set to 32.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Merge branch 'master' of github.com:xenserver/win-xenbus
Merge pull request #25 from pauldu/master

Add a minimum reservation feature to the pool code and use it in GNTTAB 

Approved-By: Ben.Chalmers@citrix.com