KVM: VMX: Fix comment to specify PID.ON instead of PIR.ON

The Outstanding Notification (ON) bit is part of the Posted Interrupt
Descriptor (PID) as opposed to the Posted Interrupts Register (PIR).
The latter is a bitmap for pending vectors.

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