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