x86emul: correct FPU code/data pointers and opcode handling

x86emul: correct FPU code/data pointers and opcode handling

Prevent leaking the hypervisor ones (stored by hardware during stub
execution), at once making sure the guest sees correct values there.
This piggybacks on the backout logic used to deal with write faults of
FPU insns.

Deliberately ignore the NO_FPU_SEL feature here: Honoring it would
merely mean extra code with no benefit (once we XRSTOR state, the
selector values will simply be lost anyway).

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