[migration] Skip virtual rtypes when adding relation definition through add_cube Virtual rtypes should be skipped as they are added dynamically on loading schema. Those are skipped by e.g. schema serialisation or add_entity_type, do the same thing here.

addressbook
basket
blog
book
calendar
comment
company
email
file
folder
i18ncontent
keyword
link
mailinglist
person
tag
timeseries
vcsfile