Add function to validate the consistency of SR-IOV config

Add function to validate the consistency of SR-IOV config

Add a function that validates that the user-provided SR-IOV
configuration is valid.  This includes basic checks that the
structure of the configuration is correct (e.g. all required
configuration nodes are present) as well as validating against
a configuration schema.

The schema validation consists of:
 - Ensuring that all required config parameters are present.
 - If the schema defines a default value for a parameter,
   adding the default value if the parameter is not set.
 - Ensuring that no parameters are specified in the config
   that are not defined in the schema.
 - Ensuring that have the correct type defined in the schema.
 - Ensuring that no configuration nodes are present for devices
   that do not exist.  For example, if 2 VFs are configured,
   then we validate that a node called VF-5 does not exist.

Differential Revision:	https://reviews.freebsd.org/D81
Reviewed by:		jhb
MFC after: 		1 month
Sponsored by:		Sandvine Inc.