Added a hosts-allow facility to TCP connections, which allows us to restrict

Added a hosts-allow facility to TCP connections, which allows us to restrict
the acceptable connections based upon a regular expression comparison with the
FQDN or the IP address.

Use the hosts-allow facility to restrict access to the relocation socket.  This
adds the configuration option xend-relocation-hosts-allow, which takes a
space-separated sequence of regular expressions.

Pass the protocol class instance through to SocketServerConnection, rather than
a new instance of that class.  This means that the new instance need not be
passed through SocketListener.acceptConnection.

Make the SocketServerConnection and SocketListener classes start their
corresponding threads and open their sockets (in the case of SocketListener)
automatically.  This means that callers do not need to save an instance locally,
just to call run() or listen() on it.  This also means that listenTCP and
listenUnix can go -- simply creating a TCPListener or UnixListener instance is
sufficient.

Signed-off-by: Ewan Mellor <ewan@xensource.com>