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: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>