[data] Add support for VirtualDisk and VirtualDiskSnapshot targets in data-export commands (e.g. vd/<name>, vds/<name>)

[AleksZimin]

What’s new
• d8 dataexport now understands two new target kinds:
• VirtualDisk → prefix vd/ or full virtualdisk/
• VirtualDiskSnapshot → prefix vds/ or full virtualdisksnapshot/

Usage examples

# Export the whole VirtualDisk
d8 data list vd/my-disk -n my-namespace

# Export a VirtualDiskSnapshot
d8 data download --publish vds/my-disk-snap 

# Explicit forms also work
d8 data list virtualdisk/my-disk/
d8 data download virtualdisksnapshot/my-disk-snap

[asergunov]

We need a description before merge