hyperv/vmbus: Allow bufrings preallocation.

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.

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D7960

hyperv/hn: Allocate bufrings in attach DEVMETHOD.

So that reinitialization, e.g. MTU change, will not fail when the system
memory is excessively fragmented.

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D7961

hyperv/vmbus: Assert that the bufring address is page aligned.

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D7962

hyperv/hn: Put debug messages under bootverbose

While I'm here, strip blank line.

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D7963