softmmu/vl: Add a "grab-mod" parameter to the -display sdl option

softmmu/vl: Add a "grab-mod" parameter to the -display sdl option

The -display sdl option is not using QAPI internally yet, and uses hand-
crafted parsing instead (see parse_display() in vl.c), which is quite
ugly, since most of the other code is using the QAPIfied DisplayOption
already. Unfortunately, the "alt_grab" and "ctrl_grab" use underscores in
their names which has recently been forbidden in new QAPI code, so
a straight conversion is not possible. While we could add some exceptions
to the QAPI schema parser for this, the way these parameters have been
designed was maybe a bad idea anyway: First, it's not possible to enable
both parameters at the same time, thus instead of two boolean parameters
it would be better to have only one multi-choice parameter instead.
Second, the naming is also somewhat unfortunate since the "alt_grab"
parameter is not about the ALT key, but rather about the left SHIFT key
that has to be used additionally when the parameter is enabled.

So instead of trying to QAPIfy "alt_grab" and "ctrl_grab", let's rather
introduce an alternative to these parameters instead, a new parameter
called "grab-mod" which can either be set to "lshift-lctrl-lalt" or to
"rctrl". In case we ever want to support additional modes later, we can
then also simply extend the list of supported strings here.

Message-Id: <20210825092023.81396-2-thuth@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>