pcp-iostat: handle transient devices more cleanly
pcp-iostat: handle transient devices more cleanly

Mark observed a pcp-iostat failure along the lines:
Traceback (most recent call last):
  File "/usr/libexec/pcp/bin/pcp-iostat", line 202, in <module>
    sts = manager.run()
  File "/usr/lib64/python3.3/site-packages/pcp/pmcc.py", line 623, in run
    self._printer.report(self)
  File "/usr/libexec/pcp/bin/pcp-iostat", line 116, in report
    rrqm = (c_rrqm[inst] - p_rrqm[inst]) / dt
KeyError: 'sda'

Although not reproduced again, the issue is understood to be related
to devices being introduced/removed between samples, and addition of
exception handling in the affected spot makes the code robust to the
anticipated failure case.

Resolves Fedora BZ #1249572.

adfernandes/pcp,aeg-aeg/pcpfans,aeg-aeg/pcpfans,adfernandes/pcp,wuliming/pcp,aeg-aeg/pcpfans,wuliming/pcp,adfernandes/pcp,wuliming/pcp,adfernandes/pcp,adfernandes/pcp,aeg-aeg/pcpfans,wuliming/pcp,adfernandes/pcp,aeg-aeg/pcpfans,adfernandes/pcp,wuliming/pcp,aeg-aeg/pcpfans,aeg-aeg/pcpfans,aeg-aeg/pcpfans,adfernandes/pcp,wuliming/pcp,wuliming/pcp,wuliming/pcp