only add devices to the smb mount model that are network shares
Summary:
previously we'd add all devices which would then cause a crash when
trying to access them ->as<NetworkShare>() because they aren't
Test Plan: have kcm open, plug in usb stick, observe no crash
Reviewers: ngraham, broulik
Reviewed By: ngraham, broulik
Subscribers: broulik, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D28276