Fix multifd with big number of channels

# gpg: Signature made Wed 04 Sep 2019 07:28:31 BST
# gpg:                using RSA key 1899FF8EDEBF58CCEE034B82F487EF185872D723
# gpg: Good signature from "Juan Quintela <quintela@redhat.com>" [full]
# gpg:                 aka "Juan Quintela <quintela@trasno.org>" [full]
# Primary key fingerprint: 1899 FF8E DEBF 58CC EE03  4B82 F487 EF18 5872 D723

* remotes/juanquintela/tags/fail-pull-request:
  multifd: Use number of channels as listen backlog
  socket: Add num connections to qio_net_listener_open_sync()
  socket: Add num connections to qio_channel_socket_async()
  socket: Add num connections to qio_channel_socket_sync()
  socket: Add backlog parameter to socket_listen

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>