hyperv/hn: Cap default # of rings to 8.

8 gives the best performance in both Azure and local Hyper-V on both
10Ge and 40Ge. More rings are still allowed by manual configuration.

Reviewed by: Dexuan Cui <decui microsoft com>
MFC after: 1 week
Sponsored by: Microsoft OSTC
Differential Revision: https://reviews.freebsd.org/D5879