Centralize kernel killswitch actions to UrfDeviceKernel.
UrfArbitrator: move software blocking for killswitch type to UrfKillswitch

Signed-off-by: Mathieu Trudel-Lapierre <mathieu.trudel-lapierre@canonical.com>
UrfDevice: make update_states generic too, and allow Devices to reimplement it.

Signed-off-by: Mathieu Trudel-Lapierre <mathieu.trudel-lapierre@canonical.com>