OvmfPkg/Xen: Fix VS2019 build issues

OvmfPkg/Xen: Fix VS2019 build issues

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3722

Fix VS2019 NOOPT build issues related to converting
a larger integer value to a smaller integer value.

Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Julien Grall <julien@xen.org>
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Jiewen Yao <Jiewen.yao@intel.com>