net/virtio-user: fix crash when detaching device

net/virtio-user: fix crash when detaching device

The rte_eth_dev.data pointer is set to a reference to a static table.
Attempting to rte_free() it leads to a panic.  For example, the
following commands result in a panic if run in testpmd

  testpmd> port attach virtio_user0,path=/dev/vhost-net,iface=test0
  testpmd> port stop 2
  testpmd> port close 2
  testpmd> port detach 2

Fixes: ce2eabdd43ec ("net/virtio-user: add virtual device")
Cc: stable@dpdk.org
Signed-off-by: Allain Legacy <allain.legacy@windriver.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>