iotests: Use new-style NBD connections

Old-style NBD is deprecated upstream (it is documented, but no
longer implemented in the reference implementation), and it is
severely limited (it cannot support structured replies, which
means it cannot support efficient handling of zeroes), when
compared to new-style NBD.  We are better off having our iotests
favor new-style everywhere (although some explicit tests,
particularly 83, still cover old-style for back-compat reasons);
this is as simple as supplying the empty string as the default
export name, as it does not change the URI needed to connect a
client to the server.  This also gives us more coverage of the
just-added structured reply code, when not overriding $QEMU_NBD
to intentionally point to an older server.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-id: 20171109221216.10248-1-eblake@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>