Skip virtual collections to avoid accidentaly loading a search collection.

Otherwise it can happen that huge amount of mails are loaded into memory just to
be discarded (2gb korgac anyone?).

I noticed this in korgac, korganizer, and kmail when trying to open invitations.

The reason were 60000 email in my Last Search folder.

This is the same fix as in kdepimlibs.

BUG: 317120