https://bugzilla.redhat.com/show_bug.cgi?id=1583623

When attaching a virtio-scsi with IOThreads for the config of a
live domain, allow the <address> to not be defined thus allowing
post parse processing to fill in the address. This allows parsing
of an individual device to succeed for attach config.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>