KVM: VMX: set vmx->emulation_required only when needed.

If emulate_invalid_guest_state=false vmx->emulation_required is never
actually used, but it ends up to be always set to true since
handle_invalid_guest_state(), the only place it is reset back to
false, is never called. This, besides been not very clean, makes vmexit
and vmentry path to check emulate_invalid_guest_state needlessly.

The patch fixes that by keeping emulation_required coherent with
emulate_invalid_guest_state setting.

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