kvm-all: kvm_irqchip_create is not expected to fail

KVM_CREATE_IRQCHIP should never fail, and so should its userspace
wrapper kvm_irqchip_create.  The function does not do anything
if the irqchip capability is not available, as is the case for PPC.

With this patch, kvm_arch_init can allocate memory and it will not
be leaked.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
