net/virtio: fix AVX512 datapath selection

net/virtio: fix AVX512 datapath selection

The AVX512 packed ring datapath selection was only done
at build time, but it should also be checked at runtime
that the CPU supports it.

This patch add a CPU flags check so that non-vectorized
path is selected at runtime if AVX512 is not supported.

Also in meson build enable vectorization only for relevant file, not for
all driver.

Fixes: ccb10995c2ad ("net/virtio: add election for vectorized path")

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Maxime Coquelin <maxime.coquelin@redhat.com>