examples/vm_power: fix build without i40e

channel_monitor.c was dependent on i40e driver being available.
This is only necessary for the TRAFFIC policy, so use #ifdef's
to not call these when i40e not available.

Fixes: f14791a8126e ("examples/vm_power_mgr: add policy to channels")
Cc: stable@dpdk.org
Signed-off-by: David Hunt <david.hunt@intel.com>
Tested-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>