OvmfPkg/XenBusDxe: Fix initialisation of gXenBusDevicePathTemplate

.. to avoid the use .member = value syntax as VS does not support it.

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@16296 6f19259b-4bc3-4df7-8a09-765794883524