hyperv/vmbus: Fix sub-channel re-open support.

For multi-channel devices, once the primary channel is closed,
a set of 'rescind' messages for sub-channels will be delivered
by Hypervisor. Sub-channel MUST be freed according to these
'rescind' messages; directly re-openning sub-channels in the
same fashion as the primary channel's re-opening does NOT work
at all.

After the primary channel is re-opened, requested # of sub-
channels will be delivered though 'channel offer' messages, and
this set of newly offered channels can be opened along side with
the primary channel.

This unbreaks the MTU setting for hn(4), which requires re-
openning all existsing channels upon MTU change.

MFC after: 1 week
Sponsored by: Microsoft OSTC
Differential Revision: https://reviews.freebsd.org/D6978