target/ppc: Don't update radix PTE R/C bits with gdbstub

target/ppc: Don't update radix PTE R/C bits with gdbstub

gdbstub shouldn't silently change guest visible state when doing address
translation. Since the R/C bits can only be updated when handling a MMU
fault, let's reuse the cause_excp flag and rename it to guest_visible.
While here drop a not very useful comment.

This was found while reading the code. I could verify that this affects
both powernv and pseries, but I failed to observe any actual bug.

Fixes: d04ea940c597 "target/ppc: Add support for Radix partition-scoped translation"
Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <158941063899.240484.2778628492106387793.stgit@bahia.lan>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>