net/virtio: restrict pointer aliasing for NEON

net/virtio: restrict pointer aliasing for NEON

Restrict pointer aliasing to allow the compiler to vectorize loops
more aggressively.

With this patch, a 9.6% improvement is observed in throughput for
the virtio-net PVP case, and a 2.4% perf improvement in throughput
for the virtio-user PVP case. All performance data are measured
on ThunderX-2 platform under the 0.001% acceptable packet loss with
2 cores on the vhost side and 1 core on the virtio side.

Signed-off-by: Joyce Kong <joyce.kong@arm.com>
Reviewed-by: Phil Yang <phil.yang@arm.com>