OvmfPkg/XenBusDxe: Add device state struct and create an ExitBoot services event.

OvmfPkg/XenBusDxe: Add device state struct and create an ExitBoot services event.

The ExitBoot event is used to disconnect from the device before the
next operating system start using them.

Change in V3:
- use the variable mMyDevice to prevent the driver from
  starting twice (if there is two different PCI devices).
- free(dev) on exit

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@16259 6f19259b-4bc3-4df7-8a09-765794883524