rteval: Enforce only one latency measurement module at a time

Latency modules will step on each other's toes if run at the same time
(on the same CPU, though that's an enhancement for later), so only
run one of them.  A priority mechanism allows selecting

Signed-off-by: Crystal Wood <crwood@redhat.com>
Signed-off-by: John Kacur <jkacur@redhat.com>