drm/i915/gvt: fix detect_host calling logic

Like other routines, intel_gvt_hypervisor_detect_host returns 0
for success.

Signed-off-by: Xiaoguang Chen <xiaoguang.chen@intel.com>
Signed-off-by: Jike Song <jike.song@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>