KVM: Add documentation on hypercalls

Thanks Alex for KVM_HC_FEATURES inputs and Jan for VAPIC_POLL_IRQ,
and Peter (HPA) for suggesting hypercall ABI addition.

Signed-off-by: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>