Add VCPU stat counters to track affinity for passthrough
interrupts.

pthru_all: Counts all passthrough interrupts whose IRQ mappings are
           in the kvmppc_passthru_irq_map structure.
pthru_host: Counts all cached passthrough interrupts that were injected
	    from the host through kvm_set_irq (i.e. not handled in
	    real mode).
pthru_bad_aff: Counts how many cached passthrough interrupts have
               bad affinity (receiving CPU is not running VCPU that is
	       the target of the virtual interrupt in the guest).
Signed-off-by: Suresh Warrier <warrier@linux.vnet.ibm.com>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>