pdm: support default image in Service,Component Individual Variations

in c361a8c3 (erp5_pdm: improve product invividual variation, 2020-03-09)
we changed to allow default images in product individual variation and modified
IndividualVariation_view to add a file upload field to upload an image, but this
form is shared with other individual variations portal types.

Without setting the default image property sheet and allowing Embedded Files as
sub-documents, saving the form was causing an error.