ovs-ofctl: Don't rely on stat() to check unix sockets.

ovs-ofctl: Don't rely on stat() to check unix sockets.

ESX supports unix sockets, but they don't manifest themselves in
file system like they do on Linux.  Instead of using stat to check
if a unix socket exist, this patch simply tries to open it instead.

Signed-off-by: Ethan Jackson <ethan@nicira.com>