Add Vhd.resize: adjust the current_size of a disk

One can create a vhd with a large virtual size and then shrink the
size to any smaller value (>=0).

Signed-off-by: David Scott <dave.scott@citrix.com>
On close, rewrite all metadata.

Previously we wrote only the trailing footer. If we rewrite everything
then we can adjust things like size and guarantee they get flushed on
close.

Signed-off-by: David Scott <dave.scott@citrix.com>
Add a simple unit test for the resize operation

Signed-off-by: David Scott <dave.scott@citrix.com>
Add the ability to change a vhd's parent (locators)

This is useful for reconstructing chains from individual pieces.

Signed-off-by: David Scott <dave.scott@citrix.com>
Add a unit test for the parent changing functionality

Signed-off-by: David Scott <dave.scott@citrix.com>
Update CHANGES for a 0.7.2

Signed-off-by: David Scott <dave.scott@citrix.com>
Bump version to 0.7.2

Signed-off-by: David Scott <dave.scott@citrix.com>
Update .gitignore

Signed-off-by: David Scott <dave.scott@citrix.com>
Merge pull request #26 from djs55/resize

Implement resize and parent updating