x86: fix slow int80 path after XPTI additions

x86: fix slow int80 path after XPTI additions

For the int80 slow path to jump to handle_exception_saved, %r14 needs to
be set up suitably for XPTI purposes. This is because of the difference
in nature between the int80 path (which is synchronous WRT guest
actions) and the exception path which is potentially asynchronous.

This is XSA-259.

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