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.
Status Job ID Name Coverage
  External
passed Cloudooo.UnitTest-Master

00:09:46

running ERP5.StandaloneDeploymentScriptTest-Debian.Buster

15972:21:13

running SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Buster

15972:21:57

running SlapOS.Eggs.UnitTest-Master.Python2

15972:18:17

running SlapOS.SoftwareReleases.IntegrationTest-Master

15972:23:03

running SlapOS.Theia.ResilienceTest-Master-ERP5

15972:23:01

running Wendelin.StandaloneDeploymentScriptTest-Debian.Buster

15972:23:03