Correctly initialise and compare pa_cvolume structs
Summary:
Previously m_volume was uninitialised, resulting in a comparison on
garbage data when first run.
By using the proper pulse audio methods we can make sure to initialise
the variable and use a faster compare than memcmp.
Reviewers: Plasma
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D3739