Accidental shutdown click if click on `Machine` guest main menu

[andry81]

Some times a Guest shutdowns because of accidental single click on both menu items in the same time: Machine and Machine->ACPI Shutdown.

This can happen because:

1. Guest main menu is at the bottom of the screen.
2. Guest main menu and submenu does overlap.
3. Mouse left button down does click on item of the main menu, but mouse left button up does click on item from a submenu.

So, because Machine and Machine->ACPI Shutdown has all the above, then some times the click happens and machine does shutdown. This bug is very annoying.

To repro:

1. Click guest Machine main menu and hold the button.
2. Move cursor to Machine->Setting... and release the button.

Expected behaviour:

1. Mouse button release has no effect on a menu or a submenu if not been initiated on the same menu.