one step toward a live migration plugin

have dirty page tracking in a separate plugin (kernel) and stop have
dirty page stuff when VM and Migration are not used.

Indeed, this plugin is only needed if vm have to be migrated live. In
that case, one should:
 1) #include "simgrid/plugins/live_migration.h"
 2) call "MSG/sg_vm_live_migration_plugin_init() right after the
    initialization of the simulation (MSG_init or s4u::Engine)