Close #105

When a VM is shutdown, a signal is triggered which is captured in the VM
live migration plugin. If the VM was currently migrating, it kills the
RX and TX actors and the actor that was used to call sg_vm_migrate in
an asynchronous way. Smart pointers on these three actors are stored
in a new extension of the VM which is created when started a migration.

A test for this feature has been added in
teshsuite/s4u/cloud-interrupt-migration which corresponds to the
scenario described in the issue