Add unix domain client server support

Add unix domain client server support

To get a properly functioning agent we will need to split the functionality
into a daemon (vdagentd, which has the rights to open the virtio device and to
create fake input devices for the mouse) and into a client (vdagent) which
runs under Xorg and thus can read / set things like the resolution and
the clipboard and talks to the spice server / client through the daemon.

Since we can have multiple xorg sessions active (through switch user for
example), the daemon supports multiple agent connections. Security
still needs to be filled in I'm afraid (see TODO).

The protocol between the 2 is "described" in vdagentd-proto.h, currently there
is only one vdagentd command, which allows vdagent to tell vdagentd the
xorg screen resolution so that it knows what resolution to use for the
fake absolute input device, and so that it can adjust that resolution
if the xorg resolution changes.

The client included in this commit is purely a test client, which
just sends a hardcoded resolution once and then sits there and does nothing.