Add support for avd configuration inheritance

Add ability to inherit and extends existing avd configurations.

```
 <rbm-avd-config id="avd1" />
      <sdcard size="1m" />
 </rbm-avd-config>

 <rbm-avd-config id="avd2" parentid="avd1 >
      <sdcard size="1m" />
 </rbm-avd-config>
```
