Add prototype vhd-format VDI.{import,export}

We add 'xe vdi-export' to the existing 'xe vdi-import' (still using HTTP GET and PUT underneath). We support format=raw as well as format=vhd. We support import and export of delta disks. For good measure we include a nice progress bar for the CLI.

The actual .vhd processing is offloaded to the vhd-tool utility.

Backported from #1782