Recently, bhyve started supporting specifying guest CPU topology.
It looks this way:

  bhyve -c cpus=C,sockets=S,cores=C,threads=T ...

The old behaviour was bhyve -c C, where C is a number of vCPUs, is
still supported.

So if we have CPU topology in the domain XML, use the new syntax,
otherwise keep the old behaviour.

Also, document this feature in the bhyve driver page.
Signed-off-by: NRoman Bogorodskiy <bogorodskiy@gmail.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>