tools/libxl: Replace deprecated -sdl option on QEMU command line

tools/libxl: Replace deprecated -sdl option on QEMU command line

"-sdl" is deprecated upstream since 6695e4c0fd9e ("softmmu/vl:
Deprecate the -sdl and -curses option"), QEMU v6.2, and the option is
removed by 707d93d4abc6 ("ui: Remove deprecated options "-sdl" and
"-curses""), in upcoming QEMU v7.1.

Instead, use "-display sdl", available since 1472a95bab1e ("Introduce
-display argument"), before QEMU v1.0.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Jason Andryuk <jandryuk@gmail.com>