net/virtio: report Tx descriptor limits in dev info

net/virtio: report Tx descriptor limits in dev info

Report max/min/align Tx descriptors limits in device info get callback.
Before calling the callback, rte_eth_dev_info_get() provides
default values of nb_min as zero and nb_max as UINT16_MAX that are
not correct for the driver, so one can't rely on them.

Signed-off-by: Ivan Ilchenko <ivan.ilchenko@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>