[IMAPx] Cannot mark-all-read in virtual Junk and Trash folders

The IMAPx provider didn't return any CamelFolderInfo when the top
folder was either the virtual Junk or the virtual Trash folder, which
caused no messages being marked are read in any of the both folders.

This had been reported at:
https://bugzilla.redhat.com/show_bug.cgi?id=1310312