Fix a regression when initial connection fails
    
    Due to changes in commit 65ef66e4, when the initial connection fails,
    virt-viewer just sat quietly and didn't indicate what was wrong. It also
    did not exit as it did before.  This is because we were using
    virt_viewer_session_spice_channel_destroy() incorrectly. This function
    was intended to be a callback that is called to clean up the VV session
    when the SpiceSession tells us that a channel has been destroyed. It
    does not actually destroy the channel, it only cleans up references to
    that channel within virt-viewer. After calling this function, the
    channel is not affected in any way. If the channel object was valid
    before calling the function, it will be valid and unchanged after
    calling the function as well.
    
    The problem is that before commit 65ef66e4, this function
    (_channel_destroy()) also had a side-effect of emitting a signal that
    made us think that the SpiceSession was disconnected when it was not.
    The application responded to this signal by exiting even though the
    session was not properly disconnected and cleaned up.
    
    We now no longer exit the application until the SpiceSession is properly
    disconnected and cleaned up. So we need to make sure that this happens
    when our initial connection fails. Therefore, when the main channel
    receives an error channel-event, we should not call
    virt_viewer_session_spice_channel_destroy(). This function should only
    be called when a channel has actually been destroyed, and the channel is
    not destroyed at this point.  We should instead explicitly disconnect
    the session, which will result in the channels being destroyed properly.
    After the session destroys all of the channels, the 'channel-destroy' signal
    will be emitted by SpiceSession, so the _channel_destroy() function will
    eventually get called by the signal handler.
    
    To make the proper use of the function more obvious, I also changed the
    function name from _channel_destroy() to _channel_destroyed() and added
    a comment.
    
    Fixes: rhbz#1666869
    Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
    Acked-by: Christophe Fergeau <cfergeau@redhat.com>