softmmu/vl: Deprecate the old grab options

The alt_grab and ctrl_grab parameter of the -display sdl option prevent
the QAPIfication of the "sdl" part of the -display option, so we should
eventually remove them. And since this feature is also rather niche anyway,
we should not clutter the top-level option list with these, so let's
also deprecate the "-alt-grab" and the "-ctrl-grab" options while we're
at it.

Once the deprecation period of "alt_grab" and "ctrl_grab" is over, we
then can finally switch the -display sdl option to use QAPI internally,
too.

Message-Id: <20210825092023.81396-3-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>