app/testpmd: use consistent vdev names

app/testpmd: use consistent vdev names

The vdev eth_bond has been renamed to net_bond.
testpmd is creating a bonding device with the old prefix.
It is changed for consistency.

The script test-null.sh was failing because using the old name
for the null vdev.

Fixes also the bonding and testpmd doc.

Fixes: 2f45703c17ac ("drivers: make driver names consistent")

Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>
Acked-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>