libxo JSON contains unescaped tab characters causing invalidity

[hayzamjs]

When running with --libxo json output, the JSON is invalid due to unescaped
tab characters in the threshold field:

"threshold":"50	100	100	0"

This causes jq to fail with:

loki :: ~ » smart ada0 -i -t -d --libxo json | jq
jq: parse error: Invalid string: control characters from U+0000 through U+001F must be escaped at line 1, column 206