iotests: Switch nbd tests to use Unix rather than TCP

iotests: Switch nbd tests to use Unix rather than TCP

Up to now, all it took to cause a lot of iotest failures was to have a
background process such as 'nbdkit -p 10810 null' running, because we
hard-coded the TCP port.  Switching to a Unix socket eliminates this
contention.  We still have TCP coverage in test 233, and that test is
more careful to not pick a hard-coded port.

Add a comment explaining where the format layer applies when using
NBD as protocol (until NBD gains support for a resize extension, we
only pipe raw bytes over the wire).

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20191114213415.23499-3-eblake@redhat.com>
[eblake: Tweak socket name per Max Reitz' review]