OvmfPkg/XenPvBlkDxe: Xen PV Block device, initial skeleton

OvmfPkg/XenPvBlkDxe: Xen PV Block device, initial skeleton

A ParaVirtualize block driver.

Change in V4:
- Replace the license by the commonly used file header text.
- Add brief description for the driver.

Change in V3:
- enable compilation for Ia32 and Ia32X64
- fix version (driver binding)

Change in V2:
- Add minimal support for controller name
- Remove stuff about BlockIo2
- Little cleanup
- Licenses and file headers
- Rename XenbusIo into XenBusIo

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16272 6f19259b-4bc3-4df7-8a09-765794883524