bhyve: Add CPU topology support

bhyve: Add CPU topology support

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: Roman Bogorodskiy <bogorodskiy@gmail.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>