virt-xml: add qemu commandline passthrough example

We forgot to mention --qemu-commandline in virt-xml.pod.
Also added an example copied from Cole's blog.

Signed-off-by: Chen Hanxiao <chenhanxiao@gmail.com>

man/virt-xml.pod

@@ -222,6 +222,8 @@ Before defining or updating the domain, show the generated XML diff and interact
 
 =item B<--panic>
 
+=item B<--qemu-commandline>
+
 These options alter the XML for a single class of XML elements. More complete documentation is found in L<virt-install(1)>.
 
 Generally these options map pretty straightforwardly to the libvirt XML, documented at L<http://libvirt.org/formatdomain.html>
@@ -337,6 +339,10 @@ Generate XML for a virtio console device and print it to stdout:
 
   # virt-xml --build-xml --console pty,target_type=virtio
 
+Add qemu command line passthrough:
+
+  # virt-xml f25 --edit --confirm --qemu-commandline="-device FOO"
+
 
 =head1 CAVEATS