[2176355] Show a reason on VM console tab when headless mode is ON

Description of problem:
Create a vm with headless mode is ON, the VNC is not available.
Click "Connect" button in the VNC tab, nothing happens. It should disable the VNC console or tell user the reason why it does not work.

Version-Release number of selected component (if applicable):

How reproducible:

Steps to Reproduce:
1. Create a VM with headless mode "ON"
2. Click "Connect" in vm VNC tab
3.

Actual results:
nothing happens

Expected results:
either disable the connect button or tell a reason after click the button

Additional info: