accel/kvm: Make "kernel_irqchip" default on

Commit 11bc4a13 ("kvm: convert "-machine kernel_irqchip" to an
accelerator property") moves kernel_irqchip property from "-machine" to
"-accel kvm", but it forgets to set the default value of
kernel_irqchip_allowed and kernel_irqchip_split.

Also cleaning up the three useless members (kernel_irqchip_allowed,
kernel_irqchip_required, kernel_irqchip_split) in struct MachineState.

Fixes: 11bc4a13 ("kvm: convert "-machine kernel_irqchip" to an accelerator property")
Reported-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Message-Id: <20191228104326.21732-1-xiaoyao.li@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>