{domain, network}_conf: disable autostart when deleting config
    
    Undefining a running, autostarted domain removes the autostart link, but
    dom->autostart is not cleared. If the domain is subsequently redefined,
    libvirt thinks it is already autostarted and will not create the link
    even if requested:
    
      # virsh dominfo example | grep Autostart
      Autostart:      enable
    
      # ls /etc/libvirt/qemu/autostart/example.xml
      /etc/libvirt/qemu/autostart/example.xml
    
      # virsh undefine example
      Domain example has been undefined
    
      # virsh define example.xml
      Domain example defined from example.xml
    
      # virsh dominfo example | grep Autostart
      Autostart:      enable
    
      # virsh autostart example
      Domain example marked as autostarted
    
      # ls /etc/libvirt/qemu/autostart/example.xml
      ls: cannot access /etc/libvirt/qemu/autostart/example.xml: No such file or directory
    
    This commit ensures dom->autostart is cleared whenever the config and
    autostart link (if present) are removed.
    
    The bridge network driver cleared this flag itself in networkUndefine.
    This commit moves this into virNetworkDeleteConfig for symmetry with
    virDomainDeleteConfig, and to ensure it is not missed in future network
    drivers.
    
    Signed-off-by: Michael Chapman <mike@very.puzzling.org>