virtchnl: update header and increase header clarity

We already have the SPDX header, so just leave a copyright notice with
an updated year and get rid of the boilerplate header (so 2002!).

In addition, update a couple of comments to clarify how the various
parts of the virtchannel header interaction work.

No functional changes.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Marek Szlosek <marek.szlosek@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>