CPU usage was calculated without taking SMP VM into account.

When we update a VM configuration while it is running, for example a change in
the network interfaces, we recreate a domain from the running XML which is fine
but this is not valid anymore when the VM is stopped.
So in the latter case, we keep an attribute on the VM indicating if we need to
recreate the VM instance from the XML when it stops.

Since a network interface cannot change.

Added options for deactivating handlers in config file.
Added deprecated option remote_execution.

Define a decorator for libvirt handlers, those can be used only when the
connection to the libvirt is active