Messing up with VM

Messing up with VM
 - move VM tracing to S4U
 - move last functions related to migration to the plugin
    - MSG_vm_create is now MSG_vm_create_migratable and tagged
      as DEPRECATED (will be removed in 3.21)
    - Assume that the live migration plugin is always loaded in JAVA
      (because all VMs are create with MSG_vm_create there)
 - add a proper destroy function to S4U VMs (shutdown first, then
   destroy)
 - pimpl_vm_ is now private (with getImpl() as accessor)
    - and use the user level interface as much as possible

Conclusion: The MSG_vm interface is now just a dummy wrapper on the
S4U interface and live migration is fully isolated in a plugin. Only
the is_migrating bool remains in VirtualMachineImpl, but is not (and
should not be) accessed or modified from outside the plugin. We can live
with that.