OvmfPkg/XenBusDxe: Add XenStore client implementation

XenStore is a key/value database, which is running on another virtual
machine. It can be accessed through shared memory. This is a client
implementation.

Change in V3:
- moving xs_wire.h from patch #1 to this patch
- fix return value of XenStoreListDirectory
- Use a timeout to print a debug message if the other side of the
  xenstore ring does not notify through the event channel.
  This is done with the new XenStoreWaitForEvent function.
- Have XenStoreReadReply check status of XenStoreProcessMessage and
  return an error if needed.
- Have XenStoreTalkv return the status of XenStoreReadReply.
- Have a loop to check for the quiescent of the response ring in the
  XenStoreInitComms function. (with a timeout of 5 seconds)
- use the recently introduced XenStore 'closing' feature.

Change in V2:
- Change comment style, from freebsd to ovmf
- Fix type of EventChannel
- Fix debug print, no more cast
- Implement XenStoreDeinit.
- Clean up comments
- Fix few codding style issue
- Add FAIL xenstore status value.

Origin: FreeBSD 10.0
License: This patch adds several files under the MIT licence.
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@16267 6f19259b-4bc3-4df7-8a09-765794883524

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