softmmu/vl: Deprecate the -sdl and -curses option

It's not that much complicated to type "-display sdl" or "-display curses",
so we should not clutter our main option name space with such simple
wrapper options and rather present the users with a concise interface
instead. Thus let's deprecate the "-sdl" and "-curses" wrapper options now.

Message-Id: <20210825092023.81396-4-thuth@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>