iotests: test NBD+TLS+iothread

iotests: test NBD+TLS+iothread

Prevent regressions when using NBD with TLS in the presence of
iothreads, adding coverage the fix to qio channels made in the
previous patch.

The shell function pick_unused_port() was copied from
nbdkit.git/tests/functions.sh.in, where it had all authors from Red
Hat, agreeing to the resulting relicensing from 2-clause BSD to GPLv2.

CC: qemu-stable@nongnu.org
CC: "Richard W.M. Jones" <rjones@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-ID: <20240531180639.1392905-6-eblake@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
(cherry picked from commit a73c99378022ebb785481e84cfe1e81097546268)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>