drm/i915/gvt/kvmgt: add vfio/mdev support to KVMGT

KVMGT leverages vfio/mdev to mediate device accesses from guest,
this patch adds the vfio/mdev support, thereby completes the
functionality. An intel_vgpu is presented as a mdev device,
and full userspace API compatibility with vfio-pci is kept.
An intel_vgpu_ops is provided to mdev framework, methods get
called to create/remove a vgpu, to open/close it, and to
access it.

Signed-off-by: Kevin Tian <kevin.tian@intel.com>
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>