KVM: PPC: Book3S HV: Tunable to disable KVM IRQ bypass

Add a  module parameter kvm_irq_bypass for kvm_hv.ko to
disable IRQ bypass for passthrough interrupts. The default
value of this tunable is 1 - that is enable the feature.

Since the tunable is used by built-in kernel code, we use
the module_param_cb macro to achieve this.

Signed-off-by: Suresh Warrier <warrier@linux.vnet.ibm.com>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>