Plus error handling.

No more global variable.

Therefore do not block next stop actions.

Refactored plugins.
Updated hypervisor libvirt event handling.
Async rpc handling.
Minor renaming for libvirt event callback.
Remove uneeded method sub_tags (in hypervisor Handler).

Create/Start/Stop/Destroy/Undefine/Export/Suspend/Resume.

TODO: Need to handle event.

Basic support.

Initialization, re-authentication...

 - list_vm now does tag filtering and vm name filtering
 - start and stop vm have name filtering 
 - stop_vm have now an optional parameter to force poweroff