Fix the case when creating a luks encrypted volume
via an xml file without 'secret' element.
libvirtd was receiving SIGSEGV, now proper error is reported for
the missing element.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1468422Signed-off-by: NKaterina Koukiou <kkoukiou@redhat.com>