OvmfPkg: Fix VS2015 warning C4459 in XenBusDxe

warning C4459: declaration of 'xs' hides global declaration.
Update code to rename local variable xs to xsp to be different.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Acked-by: Michael Kinney <michael.d.kinney@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@19116 6f19259b-4bc3-4df7-8a09-765794883524