Pass SR-IOV configuration to kernel using an nvlist

Pass all SR-IOV configuration to the kernel using an nvlist. The
main benefit that this offers is flexibility. It allows a driver
to accept any number of parameters of any type supported by the
SR-IOV configuration infrastructure with having to make any
changes outside of the driver.

It also offers the user very fine-grained control over the
configuration of the VFs -- if they want, they can have different
configuration applied to every VF.

Differential Revision: https://reviews.freebsd.org/D82
Reviewed by: jhb
MFC after: 1 month
Sponsored by: Sandvine Inc.