hyperv/vmbus: Add missing white space.

Submitted by:   QianYue You <t-youqi microsoft com>
Sponsored by:   Microsoft

hyperv/vmbus: Implement vmbus_chan_printf.

And use it for vmbus channel logging, which can log the channel
owner's name properly, instead of vmbus0.

Submitted by:   QianYue You <t-youqi microsoft com>
Sponsored by:   Microsoft

hyperv/vmbus: Reset ch_dev, once the child is deleted.

So it will not be mis-used later on, e.g. in vmbus_chan_printf().

Submitted by:   dexuan
Reported by:    dexuan
Sponsored by:   Microsoft