condition: buffer usage: validation does not check for ratio and bytes threshold

condition: buffer usage: validation does not check for ratio and bytes threshold

Buffer usage condition do not support having both ratio and bytes
threshold set.

Using `!usage->threshold_ratio.set && !usage->threshold_bytes.set` does
not error out for `threshold_ratio.set == 1` and
`usage->threshold_bytes.set == 1`

0 0 : 1
0 1 : 0
1 0 : 0
1 1 : 0

We want to check for a xnor (`usage->threshold_ratio.set == usage->threshold_bytes.set`)

0 0 : 1
0 1 : 0
1 0 : 0
1 1 : 1

We could also do 2 error check one for not set and one validating that
both are not set.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I5bfcf43bcaf9687ddf9fdebe5ace4f15bda28261