docs: schemas: Add 'seclabel' for external disk snapshot

docs: schemas: Add 'seclabel' for external disk snapshot

Allow using seclabels the same way as disk images allow it. Currently
the snapshot code copies the seclabels from the original image if no
seclabel is provided. Also there's no code change required as the
snapshot XML parser actually uses parts of the disk parser thus
seclabels are already parsed and formatted and even applied thus this is
just a formalization of our support for this.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>