libvirt xml: fix timezone syntax

Libvirt XML syntax require lowercase utc as 'basis' attribute.
This option is used only if 'timezone' property is set. And apparently
libvirt prior to 4.1.0 didn't support it for Xen at all. Which means the
setting is ignore prior to Qubes R4.1.

templates/libvirt/xen.xml

@@ -77,9 +77,9 @@
             {% if timezone == 'localtime' %}
                 <clock offset="variable" adjustment="0" basis="localtime" />
             {% elif timezone.isdigit() %}
-                <clock offset="variable" adjustment="{{ timezone }}" basis="UTC" />
+                <clock offset="variable" adjustment="{{ timezone }}" basis="utc" />
             {% else %}
-                <clock offset="variable" adjustment="0" basis="UTC" />
+                <clock offset="variable" adjustment="0" basis="utc" />
             {% endif %}
         {% else %}
             <clock offset='utc' adjustment='reset'>