drm/i915/gvt: remove unused parameter for hypervisor's host_exit call

The parameter 'void *gvt' is not used and required for hypervisor's
exit call. Even for non-merged Xen hypervisor support. So just remove it.

Reviewed-by: Yuan, Hang <hang.yuan@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>