KVM: VMX: Consider PID.PIR to determine if vCPU has pending interrupts

Commit 17e433b5 ("KVM: Fix leak vCPU's VMCS value into other pCPU")
introduced vmx_dy_apicv_has_pending_interrupt() in order to determine
if a vCPU have a pending posted interrupt. This routine is used by
kvm_vcpu_on_spin() when searching for a a new runnable vCPU to schedule
on pCPU instead of a vCPU doing busy loop.

vmx_dy_apicv_has_pending_interrupt() determines if a
vCPU has a pending posted interrupt solely based on PID.ON. However,
when a vCPU is preempted, vmx_vcpu_pi_put() sets PID.SN which cause
raised posted interrupts to only set bit in PID.PIR without setting
PID.ON (and without sending notification vector), as depicted in VT-d
manual section 5.2.3 "Interrupt-Posting Hardware Operation".

Therefore, checking PID.ON is insufficient to determine if a vCPU has
pending posted interrupts and instead we should also check if there is
some bit set on PID.PIR if PID.SN=1.

Fixes: 17e433b5 ("KVM: Fix leak vCPU's VMCS value into other pCPU")
Reviewed-by: Jagannathan Raman <jag.raman@oracle.com>
Co-developed-by: Liran Alon <liran.alon@oracle.com>
Signed-off-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>