software/kvm: fix noVNC access

Since f48a06ec noVNC is broken: the
default port for websocket is 5700 + display (and not 5900 + display).

Fix this error and add a promise to test websocket.
6 jobs for 1.0
Status Job ID Name Coverage
  External
passed ERP5.StandaloneDeploymentScriptTest-Debian.Buster

passed SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Buster

passed SlapOS.Eggs.UnitTest-Master.Python2

passed SlapOS.SoftwareReleases.IntegrationTest-Master

failed SlapOS.Theia.ResilienceTest-Master-ERP5

passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster