QMP, Introduce xen-set-global-dirty-log command. This command is used during a migration of a guest under Xen. It calls memory_global_dirty_log_start or memory_global_dirty_log_stop according to the argument pass to the command. Signed-off-by: Anthony PERARD <anthony.perard@citrix.com> Reviewed-by: Luiz Capitulino <lcapitulino@redhat.com>