OvmfPkg/XenBusDxe: Add Grant Table functions.

OvmfPkg/XenBusDxe: Add Grant Table functions.

There are used to grant access of pages to other Xen domains.

This code originaly comes from the Xen Project, and more precisely from
MiniOS.

Change in V4:
- Add license to GrantTable.h

Change in V3:
- Add a comment about the use of the BAR of the device.

Change in V2:
- Adding locks
- Redo the file header
- Add functions comment
- Add license

Signed-off-by: Steven Smith <sos22@cam.ac.uk>
Signed-off-by: Grzegorz Milos <gm281@cam.ac.uk>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16264 6f19259b-4bc3-4df7-8a09-765794883524