Should multiple vfkit instances be allowed to attach the same block device?

[vyasgun]

The introduction of NewDiskBlockDeviceStorageDeviceAttachment means that block devices can be attached to the VM but this new API allows multiple vfkit processes to attach the same device and could cause runtime issues.

Follow up issue to the PR: #373

[nirs]

Sharing same device can be done only if the device are readonly. Mounting the device the same device twice will corrupt the file system.

qemu use file locking to prevent using the same block device by more than one vm.