[Libguestfs] [PATCH v2] customize/perl_edit-c.c: Don't use internal APIs.

In v1 of this patch, there was the (small) possibility that 'g' might have been garbage collected while we were in the C function. Avoid this by passing 'g' to the function as well as the C pointer, so that 'g' is pinned as a garbage collector root [by CAMLparam5] so it cannot be collected while we're in the function. Rich.