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: Roman Bogorodskiy <bogorodskiy@gmail.com>