MFC 305801,305923,305924

MFC 305801,305923,305924

305801
    hyperv/hn: Fix some ifnet settings

    - ifnet.if_mtu does not require explicit setting.
    - ifnet.if_hdrlen must be set after ether_ifattach().

    Sponsored by:   Microsoft
    Differential Revision:  https://reviews.freebsd.org/D7873

305923
    hyperv/hn: Regroup ifnet setup code.

    While I'm here, add comment along the attach DEVMETHOD.

    Sponsored by:   Microsoft
    Differential Revision:  https://reviews.freebsd.org/D7874

305924
    hyperv/hn: Put debug message under bootverbose

    Sponsored by:   Microsoft
    Differential Revision:  https://reviews.freebsd.org/D7875

git-svn-id: svn://svn.freebsd.org/base/stable/10@307211 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f