Support specifying volume driver discard option

See: https://libvirt.org/formatdomain.html#devices

Author: nickbroon

templates/vm.xml.j2

@@ -43,7 +43,7 @@
     <emulator>{{ libvirt_vm_emulator }}</emulator>
 {% for volume in volumes %}
     <disk type='{{ volume.type | default(libvirt_volume_default_type) }}' device='{{ volume.device | default(libvirt_volume_default_device) }}'>
-      <driver name='qemu' type='{{ volume.format | default(libvirt_volume_default_format) }}'/>
+      <driver name='qemu' type='{{ volume.format | default(libvirt_volume_default_format) }}' {% if volume.discard is defined %} discard='{{ volume.discard }}' {% endif %}/>
       {% if volume.type | default(libvirt_volume_default_type) == 'file' %}
       <source file='{{ volume.file_path |default(libvirt_volume_default_images_path) }}/{{ volume.name}}'/>
       {% elif volume.type | default(libvirt_volume_default_type) == 'network' %}