ethdev: add NIC reset operation

ethdev: add NIC reset operation

This patch adds a new eth_dev layer API function rte_eth_dev_reset(),
which a DPDK application can call to reset a NIC and keep its port id
afterwards. It means that all software resources allocated in the ethdev
layer are kept, and software & hardware resources of the NIC within the
NIC's PMD are reset to a state simular to that obtained by calling the
PCI dev_uninit() and then dev_init(). This effective sequence of
dev_uninit() and dev_init() is packed into a single API function
rte_eth_dev_reset().

Please see the comments before the declaration of rte_eht_dev_reset()
in lib/librte_ether/rte_ethdev.h to get more details on why this
function is needed, what it does, when it should be called
and what an application should do after calling this function.
See also detailed explanations in the programmer's guide.

Signed-off-by: Wei Dai <wei.dai@intel.com>
Reviewed-by: Remy Horton <remy.horton@intel.com>