Add iovctl functions for validating config

Add an function to iovctl that validates the configuration against
a schema. This function is able to assume that the parser has
done most of the validation already and it's only responsible for
applying default VF values specified in the config file, confirming
that all required parameters have been set and that no invalid VF
numbers have been specified.

Differential Revision: https://reviews.freebsd.org/D85
Reviewed by: bcr
MFC after: 1 month
Sponsored by: Sandvine Inc.