net/vhost: fix socket file deleted on stop

net/vhost: fix socket file deleted on stop

If you create a vhost server device, it doesn't create the actual datagram
socket until you call .dev_start().  If you call .dev_stop() is also
deletes those sockets.  For QEMU clients, this is a problem since QEMU
doesn't know how to re-attach to datagram sockets that have gone away.

To fix this, register and unregister the datagram sockets during device
creation and removal.

Fixes: ee584e9710b9 ("vhost: add driver on top of the library")
Cc: stable@dpdk.org
Signed-off-by: Chas Williams <ciwillia@brocade.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>