bhyve: support serial type 'tcp'

bhyve: support serial type 'tcp'

In addition to the nmdm consoles, bhyve also supports a tcp console.
It's configured with:

 .. -l com1,tcp=127.0.0.1:12345

Then a user could connect to the guest console port 0 by making a tcp
connection to the host's 127.0.0.1:12345.

In the domain XML this configuration is represented as:

  <serial type='tcp'>
    <source mode='bind' host='127.0.0.1' service='12345'/>
    <target type='serial' port='0'/>
  </serial>

Also, update domain capabilities to include the TCP console support.
Unfortunately, there's no way to detect that from the bhyve binary
before trying to start a VM, so there's no capability probing for this
feature.

Signed-off-by: Roman Bogorodskiy <bogorodskiy@gmail.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>