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>
I copied it from the original case. Yes it's pointless to print the file name. but I don't think it's useful to print the content of a backup XML. :smile: