OvmfPkg/XenPvBlkDxe: Add BlockIo.

OvmfPkg/XenPvBlkDxe: Add BlockIo.

Implement the BlockIo protocol.

Change in V4:
- Replace the license by the commonly used file header text.

Change in V3:
- assert(Media->BlockSize % 512 == 0)
- Use Sector instead of Offset to issue IOs.

Change in V2:
- Remove blockIo2 headers.
- Fix few comment.
- file header, copyright
- Rewrite few comment and error messages
- No more callback
- Improving block read/write, increase to the max size in one request
  (instead of only 8pages)
- Fix lastblock when it's a cdrom
- Do uninitialisation when fail to install fail
- few comment
- Licenses

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@16274 6f19259b-4bc3-4df7-8a09-765794883524