hw/virtio: fix vhost user fails to startup when MQ

hw/virtio: fix vhost user fails to startup when MQ

 Qemu2.7~2.9 and vhost user for dpdk 17.02 release work together
to cause failures of new connection when negotiating to set MQ.
(one queue pair works well).
   Because there exist some bugs in qemu code when introducing
VHOST_USER_PROTOCOL_F_REPLY_ACK to qemu. When vhost_user_set_mem_table
is invoked to deal with the vhost message VHOST_USER_SET_MEM_TABLE
for the second time, qemu indeed doesn't send the messge (The message
needs to be sent only once)but still will be waiting for dpdk's reply
ack, then, qemu is always freezing, while DPDK is always waiting for
next vhost message from qemu.
  The patch aims to fix the bug, MQ can work well.
  The same bug is found in function vhost_user_net_set_mtu, it is fixed
at the same time.
  DPDK related patch is as following:
  http://www.dpdk.org/dev/patchwork/patch/23955/

Signed-off-by: Zhiyong Yang <zhiyong.yang@intel.com>
Cc: qemu-stable@nongnu.org
Fixes: ca525ce5618b ("vhost-user: Introduce a new protocol feature REPLY_ACK.")
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Jens Freimann <jfreiman@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>