qa/113: fix filter to select df output by fs device
qa/113: fix filter to select df output by fs device

On test-ubuntu1804-direct there is a fs mounted on /dev/sda1 and
/dev/sda15 ... when trying to select /dev/sda1 we were picking
both this one _and_ /dev/sda15 because of a prefix match.

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp