jsnbd: Disable command tracing in state hook script

Enabling 'set -x' can be useful for debugging, but as default behavior
it's basically just noise.  Thus far that noise hasn't ended up anywhere
visible because nbd-proxy redirects stderr to /dev/null when running
this script, but that may soon be changing so that actual error messages
from the script aren't lost.

Signed-off-by: Zev Weiss <zev@bewilderbeest.net>
Change-Id: Idc7d0d3fd54fd9349bf39f47c3e62a89d98b5d03