kvm创建虚拟机总是失败
~# kvm-ok
INFO: /dev/kvm exists
KVM acceleration can be used
~# kvm -m 256 -cdrom ubuntu-server.iso -drive file=server.img if=virtio index=0 -boot d -net nic -net user -nographic -vnc :0
kvm: -drive file=server.img: drive with bus=0, unit=0 (index=0) exists
请问这是什么问题?