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>
It might be better to move ```- pm_default``` and ```- pm_disabled``` to negative test part, and then let ```pm_enable``` be default for the whole test case