image create should deal w/errors returned from proxmox target

[eyberg]

https://github.com/nanovms/ops/blob/master/proxmox/proxmox_image.go#L109-L118

[eyberg]

also - attaching virtio disk has the same issue ->

https://github.com/nanovms/ops/blob/master/proxmox/proxmox_instance.go#L123-L132