examples/l3fwd-power: allow only supported environments

Currently, l3fwd-power will attempt to run even if the power env
is set to KVM, which is not supported. Fix this by preventing the
app from initializing unless the env is set to one of the supported
modes.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Tested-by: Yinan Wang <yinan.wang@intel.com>