drm/i915/gvt: Move vGPU type related code into gvt file

In this patch, all the vGPU type related code will be merged into
same gvt file and the common interface will be exposed to both
XenGT and KvmGT.

v2:
- remove the useless mdev_* gvt_ops.
  add get_gvt_attr ops for MPT module.
  intel_gvt_{init,cleanup}_vgpu_type_groups are initialized in
  gvt part. (Wang, Zhi)
- set gvt_vgpu_type_groups[i] to NULL. (Zhang,Xiong)

Signed-off-by: fred gao <fred.gao@intel.com>
Reviewed-by: Zhi Wang <zhi.a.wang@intel.com>
Signed-off-by: Zhi Wang <zhi.a.wang@intel.com>