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