OvmfPkg/XenPvBlkDxe: Add BlockFront client.

This is the code that will do the actual communication between OVMF and
a PV block backend, where the block device lives. The protocol used is
describe in the blkif.h header.

This implementation originally comes from Mini-OS, a part of the Xen
Project.

Change in V4:
- add file header to BlockFront.h (license, copyright, brief desc)

Change in V3:
- Improve comment of XenBusReadUint64.
- Moving blkif.h to this patch
  with the necessary #pragma pack(4) applied for Ia32.
- Add a note about the license in the commit message
- Add "The protocol used is describe in the blkif.h header." in the
  commit message
- Have a mandatory sector-size multiple of 512 or fail to initialize.
- use Sector instead of Offset for IO request.
  with Sector been 512-byte unit.
- print something if EventChannelNotify return an error.

Change in V2:
- trigger CoW is probably not needed on OVMF (as opposed to Mini-OS),
  removed the test.
- comments
- renamed XenbusReadInteger to XenBusReadUint64
- remove callback from IoData, use simple status instead
- return a status from the synchronus io
- Close protocol if blockfront init fail.
- fix few debug print
- Rename XenbusIo to XenBusIo
- XenPvBlkWaitForBackendState will return an error if the new backend
  states is not the expected state.
- Add the license

License: This patch adds some files which are under the MIT license.
Signed-off-by: Samuel Thibault <samuel.thibault@eu.citrix.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Jordan Justen <jordan.l.justen@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16273 6f19259b-4bc3-4df7-8a09-765794883524

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