pc_compat_2_4() doesn't exist, and we shouldn't create one. Add a
comment explaining why the function doesn't exist and why pc_compat_*()
functions are deprecated.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>