qemu: don't kill qemu process on restart if networkNotify fails
    
    Nothing that could happen during networkNotifyActualDevice() could
    justify unceremoniously killing the qemu process, but that's what we
    were doing.
    
    In particular, new code added in commit 85bcc022 (first appearred in
    libvirt-3.2.0) attempts to reattach tap devices to their assigned
    bridge devices when libvirtd restarts (to make it easier to recover
    from a restart of a libvirt network). But if the network has been
    stopped and *not* restarted, the bridge device won't exist and
    networkNotifyActualDevice() will fail.
    
    This patch changes networkNotifyActualDevice() and
    qemuProcessNotifyNets() to return void, so that qemuProcessReconnect()
    will soldier on regardless of what happens (any errors will still be
    logged though).
    
    Partially resolves: https://bugzilla.redhat.com/1442700