sdl: improve error message on fatal error

The SDL_SetVideoMode() error condition is easily triggered by a user by
simply configure a guest with a host unsupported display resolution
and attempting to enable fullscreen.  Since the error is fatal, adding
a bit of debugging help can't harm.

Sample output with this change:

 (qemu) Could not open SDL display (1280x1024x32): No video mode large enough for 1280x1024

The width x height might seem redundant as SDL also provides it in
SDL_GetError(), but I believe there are situations where it is
useful.  I.e. if there is some other SDL error.  Anyway, redundant
information in fatal error messages has never harmed a single gerbil.
Signed-off-by: NBjørn Mork <bjorn@mork.no>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

sdl.c

@@ -112,7 +112,8 @@ static void do_sdl_resize(int new_width, int new_height, int bpp)
     height = new_height;
     real_screen = SDL_SetVideoMode(width, height, bpp, flags);
     if (!real_screen) {
-        fprintf(stderr, "Could not open SDL display\n");
+	fprintf(stderr, "Could not open SDL display (%dx%dx%d): %s\n", width, 
+		height, bpp, SDL_GetError());
         exit(1);
     }
 }