qa/309: don't run if no disk.dev.read instances
qa/309: don't run if no disk.dev.read instances

Should not happen on most systems, but does on FreeBSD (lameness
of freebsd PMDA I suspect), and when it does happen, pmstore(1)
fails (not unexpected) and the QA test produces the wrong output.

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