x86/hvm: Correct the position of the %cs L/D checks

x86/hvm: Correct the position of the %cs L/D checks

Contrary to the description in the software manuals, in Long Mode, attempts to
load %cs check that D is not set in combination with L before the present flag
is checked.

This can be observed because the L/D check fails with #GP before the presence
check failes with #NP.

This change partially reverts c/s 78ff18c90 "x86: defer not-present segment
checks", taking it back to how it was in the v1 submission.

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