Add support for smartmontools 7.0 user_capacity json output

Summary:
smartmontools 7.0 reports user capacity as an object that contains bytes and blocks value, instead of being an int that just contains bytes
This should fix incorrect calculation of bad blocks threshold and incorrect display of "Overall" status

Test Plan:
run partitionmanager on a system that has smartmontools 7, open smart status
overall status should be reported correctly

Reviewers: stikonas

Reviewed By: stikonas

Differential Revision: https://phabricator.kde.org/D21311