KVM: VMX: Introduce pi_is_pir_empty() helper

Streamline the PID.PIR check and change its call sites to use
the newly added helper.

Suggested-by: Liran Alon <liran.alon@oracle.com>
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>