x86: allow grant table originating ptes to have their guest available

x86: allow grant table originating ptes to have their guest available
bits set.

In order to allow 2.6.27's get_user_pages_fast() to correctly detect
granted pages (so they would be passed to get_user_pages()), it must
be possible to tell the grant table code in Xen to set the three
available bits in the pte-s it creates.

Signed-off-by: Jan Beulich <jbeulich@novell.com>