ui/sdl: Load optional QEMU icon

ui/sdl: Load optional QEMU icon

Load an optional QEMU icon file. If there is no icon file named
qemu.bmp in QEMU's default search path, QEMU will run with
the usual system default icon.

A matching icon file  will be loaded and used by X Windows managers
or MS Windows while a QEMU instance is running.

SDL requires icon files in 32x32x4 bmp format.

Cc: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>