KVM: VMX: fix DPL during entry to protected mode

On CPUs without support for unrestricted guests DPL cannot be smaller
than RPL for data segments during guest entry, but this state can occurs
if a data segment selector changes while vcpu is in real mode to a value
with lowest two bits != 00. Fix that by forcing DPL == RPL on transition
to protected mode.

This is a regression introduced by c865c43de66dc97.

Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>