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.