virtchnl: do structure hardening

The virtchnl interface can have a bunch of "soft" defined structures
hardened by using explicit sizes for declarations, and then referring to
the enum type that uses them in a comment. None of these changes should
change any of the structure sizes.

Also, remove a duplicate line in a switch statement and let two cases
uses the same code.

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>