Don't ignore default virtual devices

Summary:
Ignoring virtual PA devices for volume control results in ignoring the
"Simultaneous output" devices. This makes it impossible to control them
with the volume keys, which is not what the user expects. As a workaround,
prefer not to ignore virtual devices that are marked as default.

Reviewers: broulik, drosca

Reviewed By: drosca

Subscribers: filipf, ngraham, cfeck, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D17784