qga: add systemd socket activation support

qga: add systemd socket activation support

AF_UNIX and AF_VSOCK listen sockets can be passed in by systemd on
startup.  This allows systemd to manage the listen socket until the
first client connects and between restarts.  Advantages of socket
activation are that parallel startup of network services becomes
possible and that unused daemons do not consume memory.

The key to achieving this is the LISTEN_FDS environment variable, which
is a stable ABI as shown here:
https://www.freedesktop.org/wiki/Software/systemd/InterfacePortabilityAndStabilityChart/

We could link against libsystemd and use sd_listen_fds(3) but it's easy
to implement the tiny LISTEN_FDS ABI so that qemu-ga does not depend on
libsystemd.  Some systems may not have systemd installed and wish to
avoid the dependency.  Other init systems or socket activation servers
may implement the same ABI without systemd involvement.

Test as follows:

  $ cat ~/.config/systemd/user/qga.service
  [Unit]
  Description=qga

  [Service]
  WorkingDirectory=/tmp
  ExecStart=/path/to/qemu-ga --logfile=/tmp/qga.log --pidfile=/tmp/qga.pid --statedir=/tmp

  $ cat ~/.config/systemd/user/qga.socket
  [Socket]
  ListenStream=/tmp/qga.sock

  [Install]
  WantedBy=default.target

  $ systemctl --user daemon-reload
  $ systemctl --user start qga.socket
  $ nc -U /tmp/qga.sock

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>