virt-viewer: Ensure to not close during migration
    
    Take a look at the shutdown event detail before killing
    the connection. Otherwise it breaks the SPICE seamless migration
    feature.
    
    Regression since commit a62827d28c6b69e90102e4c1c8043cbddad8929a
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1442929
    Signed-off-by: Pavel Grunt <pgrunt@redhat.com>
    Acked-by: Daniel P. Berrange <berrange@redhat.com>