Only log error in case of error.

When VM is not started, vnc port value is -1 in the libvirt domain XML config.

In case of an exception, the value of the tag is set to None.

When doing remote execution, it was sometimes failing with the following
exception: OSError: [Errno 10] No child processes.
The problem is when using libev default loop, the latest register a handler for
SIGCHLD, it causes a problem when Popen.communicate() tried to do a wait_pid,
the syscall is skipped because of the signal. Then when wait_pid is called
again, it failed because the process don't exist anymore.

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.