docs: bhyve: document commandline element

Document the <bhyve:commandline> element which allows
to inject custom command line arguments for bhyve.
Signed-off-by: NRoman Bogorodskiy <bogorodskiy@gmail.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>

docs/drvbhyve.html.in

@@ -460,6 +460,28 @@ Example:</p>
     &lt;/cpu&gt;
     ...
 &lt;/domain&gt;
+</pre>
+
+<h3><a id="bhyvecommand">Pass-through of arbitrary bhyve commands</a></h3>
+
+<p><span class="since">Since 5.1.0</span>, it's possible to pass additional command-line
+arguments to the bhyve process when starting the domain using the
+<code>&lt;bhyve:commandline&gt;</code> element under <code>domain</code>.
+To supply an argument, use the element <code>&lt;bhyve:arg&gt;</code> with
+the attribute <code>value</code> set to additional argument to be added.
+The arg element may be repeated multiple times. To use this XML addition, it is necessary
+to issue an XML namespace request (the special <code>xmlns:<i>name</i></code> attribute)
+that pulls in <code>http://libvirt.org/schemas/domain/bhyve/1.0</code>;
+typically, the namespace is given the name of <code>bhyve</code>.
+</p>
+<p>Example:</p>
+<pre>
+&lt;domain type="bhyve" xmlns:bhyve="http://libvirt.org/schemas/domain/bhyve/1.0"&gt;
+  ...
+  &lt;bhyve:commandline&gt;
+    &lt;bhyve:arg value='-somebhyvearg'/&gt;
+  &lt;/bhyve:commandline&gt;
+&lt;/domain&gt;
 </pre>
 
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