x86emul: generalize exception handling for rep_* hooks

x86emul: generalize exception handling for rep_* hooks

If any of those hooks returns X86EMUL_EXCEPTION, some register state
should still get updated if some iterations have been performed (but
the rIP update will get suppressed if not all of them did get handled).
This updating is done by register_address_increment() and
__put_rep_prefix() (which hence must no longer be bypassed). As a
result put_rep_prefix() can then skip most of the writeback, but needs
to ensure proper completion of the executed instruction.

While on the HVM side the VA -> LA -> PA translation process ensures
that an exception would be raised on the first iteration only, doing so
would unduly complicate the PV side code about to be added.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>