x86emul: drop "seg" parameter from insn_fetch() hook

x86emul: drop "seg" parameter from insn_fetch() hook

This is specified (and asserted for in a number of places) to always be
CS. Passing this as an argument in various places is therefore
pointless. The price to pay is two simple new functions, with the
benefit of the PTWR case now gaining a more appropriate error code.

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