check domain state before inspecting nics/disks

If there are shutoff instances,the ceilometer-inspector vill
inspect faild,and record the tedious error string in log.
The inspector of ceilometer should check the domain state before
inspecting nics and disks,and record more conciser log message in log file.

Change-Id: Id56bf75b47edf1f3b4ba77f40a48d83186d0afb6
Closes-bug: #1271454