app/testpmd: update forward engine beginning

app/testpmd: update forward engine beginning

For each forward engine, there may be some special conditions
must be met before the forwarding runs.

Adding checks for these conditions in configuring is not suitable,
because one condition may rely on multiple configurations, and the
conditions required by each forward engine is not general.

The best solution is each forward engine has a callback to check
whether these conditions are met, and then testpmd can call the
callback to determine whether the forwarding can be started.

There was a void callback 'port_fwd_begin' in forward engine,
it did some initialization for forwarding, this patch updates its
return value then we can add some checks in it to confirm whether
the forwarding can be started. In addition, this patch calls the
callback before the forwarding stats is reset and then launches the
forwarding engine.

Bugzilla ID: 797
Cc: stable@dpdk.org
Signed-off-by: Alvin Zhang <alvinx.zhang@intel.com>
Acked-by: Xiaoyun Li <xiaoyun.li@intel.com>