drm/i915/gvt: introduce host_init/host_exit to MPT

GVT host needs init/exit hooks to do some initialization/cleanup
work, e.g.: vfio mdev host device register/unregister.

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