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 …
Maybe I misunderstanding your meaning, but the code logical i saw is:
In ```LibvirtQemuConfig```, it define option_type of ```snapshot_image_format``` to ```string```, and when you call ```qemu_conf["snapshot_image_format"] = Value```, it equal to call ```set_string('snapshot_image_format', 'Value')```, and:
```
    def set_string(self, option, value):
        self[option] = '"%s"' % value  
```