Add ability to attach and link to existing disks

Create linked vmdk and add to vm with,
```
govc vm.disk.attach -disk="test1.vmdk" -vm="testvm2"
```
Default is independent (linked) persistent disk. Use -persist flag to create non-persistent disk.
```
govc vm.disk.attach -disk="test1.vmdk" -vm="testvm2" -persist=false
```
Set -link flag to false to attach dependent disk.
```
govc vm.disk.attach -disk="test1.vmdk" -vm="testvm2" -link=false
```
Please check for error here.