x86/emul: Correct the behaviour of pop %ss and interrupt shadowing

x86/emul: Correct the behaviour of pop %ss and interrupt shadowing

The mov_ss retire flag should only be set once load_seg() has returned
success.  In particular, it should not be set if an exception occured when
trying to load %ss.

_hvm_emulate_one(), currently the sole user of mov_ss, only consideres it in
the case that x86_emulate() returns X86EMUL_OKAY, so this bug isn't actually
exposed to guests.

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