The code (since 448be8f7) useds 'vport_ops' in XML.
Later commit cc17f092 added schema for 'vports_ops' (extra 's').
Fix the schema and the corresponding docs.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2121262


Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>