hyperv/hn: Move TX tasks' draining into hn_suspend().

This prepares to consolidate hn_stop() and netvsc_detach().

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D8055

hyperv/hn: Consolidate hn_init() and hn_stop()

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D8056