qemu: capabilities: Explain that QEMU_CAPS_CHARDEV_JSON will be used in tests only

qemu: capabilities: Explain that QEMU_CAPS_CHARDEV_JSON will be used in tests only

I've added that capability a long time ago when I was converting various
stuff to use JSON but the support in '-chardev' didn't yet materialize.

Fix the comment to make that clear and also that it'll be used in tests
for the upcoming refactor of the chardev code (so that we can validate
generator against the schema even if that doesn't yet work).

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>