bhyve supports 'gop' video device that allows clients to connect
to VMs using VNC clients. This commit adds support for that to
the bhyve driver:

 - Introducr 'gop' video device type
 - Add capabilities probing for the 'fbuf' device that's
   responsible for graphics
 - Update command builder routines to let users configure
   domain's VNC via gop graphics.
Signed-off-by: NRoman Bogorodskiy <bogorodskiy@gmail.com>