[virt-tools-list] Improve window close behaviour with multiple screens

[Christophe Fergeau]

Hey,

Currently, if the guest OS has multiple screens, virt-viewer shows
one window per-screen. When the user closes one of these windows,
the corresponding screen is disabled in the guest OS. This means that
if one closes each virt-viewer window one by one and then restart
virt-viewer, only one screen will be used while initially the VM
used multiple screens.

This patch series changes this by only showing a close button on the
primary screen window, and by causing virt-viewer to exit (same as
File/Quit) when the close button is pressed rather than only disabling
the screen.

Christophe