hyperv/hn: Pull vmbus channel open up.

While I'm here, pull up the channel callback related code too.

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D7805

hyperv/hn: Push RXBUF size adjustment down.

It is not used in other places.

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D7806

hyperv/hn: Factor out function to do NVS initialization.

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D7807

hyperv/hn: Pass MTU around.

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D7808