x86emul: correct and extend IDT entry checks

x86emul: correct and extend IDT entry checks

In order to pre-determine whether a fault will occur upon software
interrupt injection, it is not sufficient to just check P and DPL. Do
at least all the checks on the IDT entry itself, and in particular do
the #NP check last. The checks for the new CS (and perhaps SS) are left
out for now, though.

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