examples/vhost: rename --dev-basename to --socket-file

examples/vhost: rename --dev-basename to --socket-file

In examples/vhost, "dev-basename" is a program option, which is to set
the vhost-net socket used by vhost-user, or the character device used
by vhost-cuse. Since vhost-cuse should be dropped, and "dev-basename"
is not a suitable name for the vhost-net socket. Therefore, this patch
is to change this option name for examples/vhost.

Signed-off-by: Jiayu Hu <jiayu.hu@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>