virtio: fix crash in statistics functions

virtio: fix crash in statistics functions

This initialisation of nb_rx_queues and nb_tx_queues has been removed
from eth_virtio_dev_init.

The nb_rx_queues and nb_tx_queues were being initialised in
eth_virtio_dev_init before the tx_queues and rx_queues arrays were
allocated.

The arrays are allocated when the ethdev port is configured and the
nb_tx_queues and nb_rx_queues are initialised.

If any of the following functions were called before the ethdev
port was configured there was a segmentation fault because
rx_queues and tx_queues were NULL:

rte_eth_stats_get
rte_eth_stats_reset
rte_eth_xstats_get
rte_eth_xstats_reset

Fixes: 823ad647950a ("virtio: support multiple queues")
Signed-off-by: Bernard Iremonger <bernard.iremonger@intel.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>