Make emulator_pio_in_out operate directly on the provided buffer
as long as PIO is handled inside KVM.

For input operations, this means that, in the case of in-kernel
PIO, __emulator_pio_in() does not have to be always followed
by complete_emulator_pio_in().  This affects emulator_pio_in() and
kvm_sev_es_ins(); for the latter, that is why the call moves from
advance_sev_es_emulated_ins() to complete_sev_es_emulated_ins().

For output, it means that vcpu->pio.count is never set unnecessarily
and there is no need to clear it; but also vcpu->pio.size must not
be used in kvm_sev_es_outs(), because it will not be updated for
in-kernel OUT.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>