core-admin/templates/libvirt/devices/block.xml

<disk type="block" device="{{ options.get('devtype', 'disk') }}">
    <driver name="phy" />
    <source dev="{{ device.device_node }}" />
    {%- if 'frontend-dev' in options %}
        <target dev="{{ options.get('frontend-dev') }}" />
    {%- elif options.get('devtype', 'disk') == 'cdrom' and not vm.kernel %}
        <!-- prefer xvdd for CDROM -->
        <target dev="xvdd" />
    {%- else %}
        <target dev="xvd{{dd[i]}}" />
        {% set i = i + 1 %}
    {%- endif %}

    {%- if options.get('read-only', 'no') == 'yes' %}
        <readonly />
    {%- endif %}

    {%- if device.backend_domain.name != 'dom0' %}
    <backenddomain name="{{ device.backend_domain.name }}" />
    {%- endif %}
</disk>