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>
This func only be called 1 time, and the logical is sample, maybe you can just do it in the calling place, using a ```lambda``` function.