guest agent: qemu-ga daemon

This is the actual guest daemon, it listens for requests over a
virtio-serial/isa-serial/unix socket channel and routes them through
to dispatch routines, and writes the results back to the channel in
a manner similar to QMP.

A shorthand invocation:

  qemu-ga -d

Is equivalent to:

  qemu-ga -m virtio-serial -p /dev/virtio-ports/org.qemu.guest_agent.0 \
          -f /var/run/qemu-ga.pid -d

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Luiz Capitulino <lcapitulino@gmail.com>