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: NEric Blake <eblake@redhat.com>
Reviewed-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>