Added sethostname command/feature

Hi Ravi, 
I added a new command to sethostname in agent. Till xen6.0 PV tools was taking care of setting the hostname. 
in Xen6.1 PV tools this feature has been removed. This change is to over come that.

https://bugs.launchpad.net/nova/+bug/1182934

Just thought of running it by you before changing the version etc. I will send one more pull request with version and other changes.