add control so that a vm will not start on a starting hypervisor if it's
already running in another hypervisor.
Change of previous behaviour : when an hypervisor disconnects or times
out, we do not put the vms back in the parking anymore. Instead, we
change the concept of "parking" to also include the vms which are
associated with an hypervisor which is not running. This may be refined
later and will need some ui work.