KVM: Portability: Add two hooks to handle kvm_create and destroy vm

Add two arch hooks to handle kvm_create_vm and kvm destroy_vm. Now, just
put io_bus init and destory in common.

Signed-off-by: Zhang Xiantao <xiantao.zhang@intel.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>