skip over supposed network shares that aren't

Summary:
we explicitly request listFromType(Solid::DeviceInterface::NetworkShare)
but apparently still can get devices that aren't actually of that type.
add an explicit type check to guard against bogus shares coming out of
listFromType.

CCBUG: 419220

Test Plan: mount an overlay (I am guessing that's what the problem with kubuntu is), observe no crash on init

Reviewers: ngraham

Reviewed By: ngraham

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28275