Add support for vgaconf driver configuration. In domain xml it looks like
this:

  <video>
    <driver vgaconf='io|on|off'>
    <model .../>
  </video>

It was added with bhyve gop video in mind to allow users control how the
video device is exposed to the guest, specifically, how VGA I/O is
handled.

One can refer to the bhyve manual page to get more detailed description
of the possible VGA configuration options:

https://www.freebsd.org/cgi/man.cgi?query=bhyve&manpath=FreeBSD+12-current

The relevant part could be found using the 'vgaconf' keyword.

Also, add some tests for this new feature.
Signed-off-by: NRoman Bogorodskiy <bogorodskiy@gmail.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>