Specify VIR_DOMAIN_XML_SECURE flag when getting XML from libvirt, fixes #5

According to the docs you are supposed to use VIR_DOMAIN_XML_MIGRATABLE
instead but there seems to be a bug in libvirt:
  https://bugzilla.redhat.com/show_bug.cgi?id=994364
Replace MIGRATE_UNSAFE (fixes #11) with MIGRATE_COMPRESSED (fixes #6).