bhyve: add vga configuration for video driver

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: Roman Bogorodskiy <bogorodskiy@gmail.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>

docs/formatdomain.html.in

@@ -6156,7 +6156,14 @@ qemu-kvm -net nic,model=? /dev/null
           <dt>virtio options</dt>
           <dd>
           <a href="#elementsVirtio">Virtio-specific options</a> can also be
-          set. (<span class="since">Since 3.5.0</span>)
+          set (<span class="since">Since 3.5.0</span>)
+          </dd>
+          <dt>VGA configuration</dt>
+          <dd>
+          Control how the video devices exposed to the guest using the
+          <code>vgaconf</code> attribute which takes the value "io", "on" or "off".
+          At present, it's only applicable to the bhyve's "gop" video model type
+          (<span class="since">Since 3.5.0</span>)
           </dd>
         </dl>
       </dd>

docs/schemas/domaincommon.rng

@@ -3220,7 +3220,18 @@
     <element name="video">
       <optional>
         <element name="driver">
-          <ref name="virtioOptions"/>
+          <optional>
+            <ref name="virtioOptions"/>
+          </optional>
+          <optional>
+            <attribute name="vgaconf">
+              <choice>
+                <value>io</value>
+                <value>on</value>
+                <value>off</value>
+              </choice>
+            </attribute>
+          </optional>
         </element>
       </optional>
       <optional>