dompmsuspend: Add VM S3/S4 enable/disable test

1) Test enable and disable VM's S3/S4 capabilities in pm tag.
2) Update test case to check standard error message.

Signed-off-by: Hao Liu <hliu@redhat.com>
IMO in this case it is not very intuitive to just classify test cases into two categories "positive" and "negative" only by whether dompmsuspend command successfully ran. In this way, the "paused" and "shutoff" options should be categorised as negative too. Then this config file will either become too complicate to handle or easy to miss some checking points. 

My thought is just use config file to create a testing matrix to cover all possible combination in an intuitive way. Then write the expected result in the python code programmatically. Do you agree with me?