x86emul: correct loading of %ss

x86emul: correct loading of %ss

- Instead of #NP, #SS needs to be raised for non-present descriptors.
- Loading a null selector is fine in 64-bit mode at CPL != 3, as long
  as RPL == CPL.
- Don't lose the low two selector bits on null selector loads (also
  applies to %ds, %es, %fs, %gs, and LDTR).

Since we need CPL earlier now, also switch to using get_cpl() (instead
of open coding it).

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