codegen: Allow to specify C type for index variable
codegen: Allow to specify C type for index variable

This is to prepare to generate the wireshark dissector which uses
glib types instead of the newer C ones (for compatibility with some
compilers).

Signed-off-by: Frediano Ziglio <55d48b080b2e443e395cde84d2c83b135a4ff48e@redhat.com>

SPICE/spice-common,SPICE/spice-common,fgouget/spice-common,SPICE/spice-common,fgouget/spice-common,fgouget/spice-common,fgouget/spice-common