x86: defer not-present segment checks

x86: defer not-present segment checks

Following on from commits 5602e74c60 ("x86emul: correct loading of
%ss") and bdb860d01c ("x86/HVM: correct segment register loading during
task switch") the point of the non-.present checks needs to be refined:
#NP (and its #SS companion), other than suggested by the various
instruction pages in Intel's SDM, gets checked for only after all type
and permission checks. The only checks getting done even later are the
long mode specific ones for system descriptors (which we don't support
yet) and 64-bit code segments (i.e. anything touching other than the
attribute byte).

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