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