i915/gvt: remove unused xen bits

No Xen support anywhere here.  Remove a dead declaration and an unused
include.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200404094101.672954-4-hch@lst.de