examples/power: fix ack for enable/disable turbo

examples/power: fix ack for enable/disable turbo

When a VM sends a command through virtio-serial to enable/disable
turbo, it is successfully enabled or disabled, yet the response to the
VM is NACK. This is because all the library frequency change APIs return
1 for success (change in frequency), 0 for success (no change in
frequency) and -1 for failure. However the turbo enable/disable APIs just
return 0 for success and -1 for failure.

Fix the handling of the return code to treat ">= 0" as success, and
send an ACK. Only send NACK when < 0 (failure).

Fixes: 0de94bcac7fc ("examples/vm_power: send confirmation cmd to guest")
Cc: stable@dpdk.org
Signed-off-by: David Hunt <david.hunt@intel.com>
Acked-by: Lei Yao <lei.a.yao@intel.com>