New optional parameter "keyfile" for ssh transport allows the user to select
the private key to be used to authenticate to the remote host.

When unregistering an I/O callback from a virNetSocket object,
there is still a chance that an event may come in on the callback.
In this case it is possible that the virNetSocket might have been
freed already. Make use of a virFreeCallback when registering
the I/O callbacks and hold a reference for the entire time the
callback is set.

* src/rpc/virnetsocket.c: Register a free function for the
  file handle watch
* src/rpc/virnetsocket.h, src/rpc/virnetserverservice.c,
  src/rpc/virnetserverclient.c, src/rpc/virnetclient.c: Add
  a free function for the socket I/O watches

Set StrictHostKeyChecking=no to auto-accept new ssh host keys if the
no_verify extra parameter was specified.  This won't disable host key
checking for already known hosts.  Includes a test and documentation.

Allow RPC servers to advertise themselves using MDNS,
via Avahi

* src/rpc/virnetserver.c, src/rpc/virnetserver.h: Allow
  registration of MDNS services via avahi
* src/rpc/virnetserverservice.c, src/rpc/virnetserverservice.h: Add
  API to fetch the listen port number
* src/rpc/virnetsocket.c, src/rpc/virnetsocket.h: Add API to
  fetch the local port number
* src/rpc/virnetservermdns.c, src/rpc/virnetservermdns.h: Represent
  an MDNS advertisement

This extends the basic virNetSocket APIs to allow them to have
a handle to the TLS/SASL session objects, once established.
This ensures that any data reads/writes are automagically
passed through the TLS/SASL encryption layers if required.

* src/rpc/virnetsocket.c, src/rpc/virnetsocket.h: Wire up
  SASL/TLS encryption

Introduces a simple wrapper around the raw POSIX sockets APIs
and name resolution APIs. Allows for easy creation of client
and server sockets with correct usage of name resolution APIs
for protocol agnostic socket setup.

It can listen for UNIX and TCP stream sockets.

It can connect to UNIX, TCP streams directly, or indirectly
to UNIX sockets via an SSH tunnel or external command

* src/Makefile.am: Add to libvirt-net-rpc.la
* src/rpc/virnetsocket.c, src/rpc/virnetsocket.h: Generic
  sockets APIs
* tests/Makefile.am: Add socket test
* tests/virnetsockettest.c: New test case
* tests/testutils.c: Avoid overriding LIBVIRT_DEBUG settings
* tests/ssh.c: Dumb helper program for SSH tunnelling tests