MFC 296291,301109

MFC 296291,301109

296291
    hyperv/chan: Factor out the vcpu setting

    And use it for cpu0 assignment; it does not sound right to assume that
    cpu0 maps to vcpu0.  And this factored out function will be exposed to
    drivers, if driver specific CPU binding is needed, e.g. hn(4).

    Move default cpu select after saving channel offer message. This makes
    sure that all useful information of the channel has been setup.

    MFC after:  1 week
    Sponsored by:       Microsoft OSTC
    Differential Revision:      https://reviews.freebsd.org/D5504

301109
    hyperv/channel: Only cpu0 is supported as channel target cpu on WIN7

    MFC after:  1 week
    Sponsored by:       Microsoft OSTC

git-svn-id: svn://svn.freebsd.org/base/stable/10@301916 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f