bhyve: add video support

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>

docs/formatdomain.html.in

@@ -5891,8 +5891,9 @@ qemu-kvm -net nic,model=? /dev/null
         <p>
           The <code>model</code> element has a mandatory <code>type</code>
           attribute which takes the value "vga", "cirrus", "vmvga", "xen",
-          "vbox", "qxl" (<span class="since">since 0.8.6</span>) or
+          "vbox", "qxl" (<span class="since">since 0.8.6</span>),
           "virtio" (<span class="since">since 1.3.0</span>)
+          or "gop" (<span class="since">since 3.2.0</span>)
           depending on the hypervisor features available.
         </p>
         <p>

docs/schemas/domaincommon.rng

@@ -3166,6 +3166,7 @@
                 <value>xen</value>
                 <value>vbox</value>
                 <value>virtio</value>
+                <value>gop</value>
               </choice>
             </attribute>
             <group>