OvmfPkg/XenBusDxe: fix VS2010 build failures

This patch contain only type cast.

Contributed-under: TianoCore Contribution Agreement 1.0

Signed-off-by: Scott Duplichan <scott@notabs.org>

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>

Build-tested-by: Scott Duplichan <scott@notabs.org>

Acked-by: Laszlo Ersek <lersek@redhat.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16392 6f19259b-4bc3-4df7-8a09-765794883524