Protect Accelio queue depth (minimal flow control).

The guard is as proposed by Vu Pham, but per xio_connection,
and considering batches.  Increment happens only if xio_send_msg
succeeded, decrement in on_ms_ow_send_complete and on_msg_error.
Note that we don't need atomics because counters are touched
only in the correct portal thread.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>