drm/i915/gvt: hold reference of VFIO group during opening of vgpu

hold reference count of the VFIO group for each vgpu at vgpu opening and
release the reference at vgpu releasing.

Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Reviewed-by: Zhenyu <Wang&lt;zhenyuw@linux.intel.com>
Signed-off-by: Zhenyu <Wang&lt;zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200313031025.7936-1-yan.y.zhao@intel.com