ui: refactor code for determining if an update should be sent to the client
    
    The logic for determining if it is possible to send an update to the client
    will become more complicated shortly, so pull it out into a separate method
    for easier extension later.
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
    Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
    Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
    Message-id: 20171218191228.31018-9-berrange@redhat.com
    Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
    (cherry picked from commit 0bad834228b9ee63e4239108d02dcb94568254d0)
    Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>