app/testpmd: add port reset command

When PF is reset, a message will show it and all its
VF need to be reset.
User can run the command "port reset port_id"
to reset the VF port and to keep same port id without
any configuration. Then user can run "port stop port_id"
and "port start port_id" to reconfigure its forwarding
mode and parameters as previous ones.
To avoid crash, current forwarding should be stopped
before running "port reset port_id".

Signed-off-by: Wei Dai <wei.dai@intel.com>
Tested-by: Yuan Peng <yuan.peng@intel.com>
Acked-by: Jingjing Wu <jingjing.wu@intel.com>