hyperv/vmbus: Allow bufrings preallocation.

[sepherosa_gmail.com]

Summary

The assumption that the channel is only opened upon synthetic device
attach time no longer holds, e.g. Hyper-V network device MTU changes.
We have to allow device drivers to preallocate bufrings, e.g. in
attach DEVMETHOD, to prevent bufring allocation failure once the
system memory is fragmented after running for a while.