netdev: Add optional qfill output parameter to rxq_recv()

netdev: Add optional qfill output parameter to rxq_recv()

If the caller provides a non-NULL qfill pointer and the netdev
implemementation supports reading the rx queue fill level, the rxq_recv()
function returns the remaining number of packets in the rx queue after
reception of the packet burst to the caller. If the implementation does
not support this, it returns -ENOTSUP instead. Reading the remaining queue
fill level should not substantilly slow down the recv() operation.

A first implementation is provided for ethernet and vhostuser DPDK ports
in netdev-dpdk.c.

This output parameter will be used in the upcoming commit for PMD
performance metrics to supervise the rx queue fill level for DPDK
vhostuser ports.

Signed-off-by: Jan Scheurich <jan.scheurich@ericsson.com>
Acked-by: Billy O'Mahony <billy.o.mahony@intel.com>
Signed-off-by: Ian Stokes <ian.stokes@intel.com>