Gnttab and Gntshr may allocate pages or not

One style of usage involves us passing in buffers and grants,
while the other involes us being returned buffers. We support
both kinds.

Signed-off-by: David Scott <dave.scott@citrix.com>
bump to 2.0.0
Merge pull request #11 from avsm/minios-gntmap

Gnttab and Gntshr may allocate pages or not