i386/kvm: do not zero out segment flags if segment is unusable or not present

i386/kvm: do not zero out segment flags if segment is unusable or not present

This is a fix for the problem [1], where VMCB.CPL was set to 0 and interrupt
was taken on userspace stack.  The root cause lies in the specific AMD CPU
behaviour which manifests itself as unusable segment attributes on SYSRET[2].

Here in this patch flags are not touched even segment is unusable or is not
present, therefore CPL (which is stored in DPL field) should not be lost and
will be successfully restored on kvm/svm kernel side.

Also current patch should not break desired behavior described in this commit:

4cae9c97967a ("target-i386: kvm: clear unusable segments' flags in migration")

since present bit will be dropped if segment is unusable or is not present.

This is the second part of the whole fix of the corresponding problem [1],
first part is related to kvm/svm kernel side and does exactly the same:
segment attributes are not zeroed out.

[1] Message id: CAJrWOzD6Xq==b-zYCDdFLgSRMPM-NkNuTSDFEtX=7MreT45i7Q@mail.gmail.com
[2] Message id: 5d120f358612d73fc909f5bfa47e7bd082db0af0.1429841474.git.luto@kernel.org

Signed-off-by: Roman Pen <roman.penyaev@profitbricks.com>
Signed-off-by: Mikhail Sennikovskii <mikhail.sennikovskii@profitbricks.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Michael Chapman <mike@very.puzzling.org>
Cc: qemu-devel@nongnu.org
Message-Id: <20170601085604.12980-1-roman.penyaev@profitbricks.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>