kvm: x86: Fix DPL write back of segment registers

The DPL is stored in the flags and not in the selector. In fact, the RPL
may differ from the DPL at some point in time, and so we were corrupting
the guest state so far.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
