Parsing xen-xm format configuration will fail if UUID is not
specified, e.g.

virsh domxml-from-native xen-xm some-config-without-uuid
error: internal error parsing xm config failed

Initially I thought to skip parsing the UUID in xenParseXM() when
not present in the configuration, but this results in a UUID of
all zeros since it is never set

virsh domxml-from-native xen-xm /tmp/jim/bug-773621_pierre-test
<domain type='xen'>
  <name>test</name>
  <uuid>00000000-0000-0000-0000-000000000000</uuid>
  ...

which certainly can't be correct since this is the UUID the xen
tools use for dom0.

This patch takes the approach of generating a UUID when it is not
specified in the configuration.