qcow2: truncate the tail of the image file after shrinking the image

qcow2: truncate the tail of the image file after shrinking the image

Now after shrinking the image, at the end of the image file, there might be a
tail that probably will never be used. So we can find the last used cluster and
cut the tail.

Signed-off-by: Pavel Butsykin <[email protected]>
Reviewed-by: John Snow <[email protected]>
Message-id: 20170929121613[email protected]
Signed-off-by: Max Reitz <[email protected]>