doc: fix parameter of virtio-user for container

doc: fix parameter of virtio-user for container

Update the "Virtio_user for Container Networking" doc, add the
"--file-prefix" option to testpmd in host and container to avoid
hugepage config file conflict.

Fixes: 50665deebda0 ("doc: add guide to use virtio-user for container networking")
Cc: stable@dpdk.org
Signed-off-by: Yong Wang <wang.yong19@zte.com.cn>
Acked-by: John McNamara <john.mcnamara@intel.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>