2D Video Acceleration Error Message Should detect platform first

[Jon Polfer]

Host: Windows XP SP3.
Guest: Debian Linux

Problem: When selecting 2D Video Acceleration, the error message presentation priority is non-helpful.

Steps To Reproduce

1. Start VirtualBox.
2. For your Debian Linux image, click Settings.
3. Click Display.
4. Set the amount of video memory to 16MB.
   -> No message is displayed in the lower information pane.
5. Click Enable 2D Video Acceleration.
   -> A message in the lower information pane appears stating that "On the Display page, you have assigned less than 20MB of video memory which is the minimum amount required for HD Video to be played efficiently."
6. Increase the amount of video memory to 20MB.
   -> A message appears in the lower information pane stating that "On the Display page, you have 2D Video Acceleration enabled. As 2D Video Acceleration is supported for Windows guests only, this feature will be disabled."

Expected behavior: I would expect the system to warn me about how the option is only valid for certain guest operating systems first in priority, and then warn me about the HD video problem. Thus, I would expect the Windows support for guests only message to appear instead of the HD Video warning.

Actual behavior: One has to adjust video memory before discovering that the 2D Video Acceleration option isn't supported for non-Windows hosts.