OvmfPkg/XenBusDxe: Add support to make Xen Hypercalls.

OvmfPkg/XenBusDxe: Add support to make Xen Hypercalls.

Change in V4:
- Replace the license by the commonly used file header text.
- add file header to XenHypercall.h (license, copyright, brief desc)

Change in V3:
- adding IA32 support. (not reviewed yet)
  both XenBusDxe/Ia32/hypercall.{S,asm} file are new

Change in V2:
- file header, copyright
- Add License
- Add push/pop instruction.
- fix types
- Comment of exported functions
- Improve coding style
- Add error handling in the main init function (of the drivers)
- Comment assembly

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>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16260 6f19259b-4bc3-4df7-8a09-765794883524