In order to use vmbuilder (in my case python-vm-builder 0.12.4) to generate fully functional Ubuntu kvm-images, the following adjustments to the libvirt-template of vmbuilder have to be made in order to work.
First, make a dir-copy of /etc/vmbuilder/libvirt/ to a location that suits you and use it by handing over the --templates-option to vmbuilder.
The following lines of libvirt/libvirtxml.tmpl have to be changed:
#if $mac
<mac address='$mac'/>
#end if
has to be moved just in front of
</interface>
The line
<graphics type='vnc' port='-1' listen='127.0.0.1'/>
has to become
<graphics type='vnc' port='-1' listen='127.0.0.1' keymap='de'/>
Add the following lines right before the closing devices-tag:
<serial type='pty'>
<target port='0'/>
</serial>