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>