The fd_is_socket() helper method is useful in a few places, so put it in
the common sockets code. Make the code more compact while moving it.

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>