Abstract access to mdev_device so that we can define which interfaces
are public rather than relying on comments in the structure.

Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: Zhi Wang <zhi.a.wang@intel.com>
Signed-off-by: NAlex Williamson <alex.williamson@redhat.com>
Reviewed-by: NJike Song <jike.song@intel.com>
Reviewed by: Kirti Wankhede <kwankhede@nvidia.com>