QEMU_IFLA_BR_MULTI_BOOLOPT has been added to the wrong function
host_to_target_slave_data_bridge_nlattr(). Move it to
host_to_target_data_bridge_nlattr().

This fixes following error:
  Unknown QEMU_IFLA_BR type 46

Fixes: 61b463fb ("linux-user: add new netlink types")
Message-Id: <20190626150855.27446-1-laurent@vivier.eu>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>