Virtualization macOS 12.0 Beta 2 to Beta 3 API Differences

Removed -[VZDiskImageStorageDeviceAttachment initWithURL:readOnly:synchronizationMode:error:]

Added VZDiskImageCachingMode

Added VZDiskImageCachingModeAutomatic

Added VZDiskImageCachingModeUncached

Added VZDiskImageCachingModeCached

Added -[VZDiskImageStorageDeviceAttachment initWithURL:readOnly:cachingMode:synchronizationMode:error:]

Added VZDiskImageStorageDeviceAttachment.cachingMode