snapshot_create_as: Add more test cases

Add reuse external diskonly negative case
Add "--quiesce" without unix channel negative case
Add external memory snapshot compressing cases 
Add test case for bug 872292 …
@will-Do It works parse string and could be set in conf, but in conf you need specify it with quote "", or libvirtd will fail to start, when with int value, you can write direct to conf.