vhd-tool: move to release v0.6.4_with_nice to get the sparse_dd (io)nice options.

Signed-off-by: Thomas Sanders <thomas.sanders@citrix.com>
Merge pull request #3 from thomassa/clw-vhd-tool-0.6.4-with-nice

vhd-tool: move to release v0.6.4_with_nice to get the sparse_dd (io)nice...