virtchnl: Add a macro to check the size of a union

This patch adds a macro to check if the size of a union is correct.
It throws a divide by zero error if the union is not of the correct
size.

Signed-off-by: Harshitha Ramamurthy <harshitha.ramamurthy@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>