MFC 305578-305581

MFC 305578-305581

305578
    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

305579
    hyperv/hn: Push RXBUF size adjustment down.

    It is not used in other places.

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

305580
    hyperv/hn: Factor out function to do NVS initialization.

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

305581
    hyperv/hn: Pass MTU around.

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

git-svn-id: svn://svn.freebsd.org/base/stable/10@307200 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f