Some tags were returning None because of bad decorator.

When the node is disconnected from the cc-server, it keeps it internal state
until the connection pass, then in case the same role is returned by the
cc-server, it just reregister its objects and tags, else it remove the previous
main plugin and recreate a new one corresponding to the adequate role.

Support for registering/unregistering sub object to the cc-server.
Support for tag lifecycle handling (libev), was previously managed by plugins.

Try to connect to libvirt while no exception is thrown.

Clean global variable for libvirt connection.

In case of error.

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).