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