virNetSocketNewConnectUNIX: Use flocks when spawning a daemon
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1200149
    
    Even though we have a mutex mechanism so that two clients don't spawn
    two daemons, it's not strong enough. It can happen that while one
    client is spawning the daemon, the other one fails to connect.
    Basically two possible errors can happen:
    
      error: Failed to connect socket to '/home/mprivozn/.cache/libvirt/libvirt-sock': Connection refused
    
    or:
    
      error: Failed to connect socket to '/home/mprivozn/.cache/libvirt/libvirt-sock': No such file or directory
    
    The problem in both cases is, the daemon is only starting up, while we
    are trying to connect (and fail). We should postpone the connecting
    phase until the daemon is started (by the other thread that is
    spawning it). In order to do that, create a file lock 'libvirt-lock'
    in the directory where session daemon would create its socket. So even
    when called from multiple processes, spawning a daemon will serialize
    on the file lock. So only the first to come will spawn the daemon.
    
    Tested-by: Richard W. M. Jones <rjones@redhat.com>
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>