qemu-img: add --shrink flag for resize

qemu-img: add --shrink flag for resize

The flag is additional precaution against data loss. Perhaps in the future the
operation shrink without this flag will be blocked for all formats, but for now
we need to maintain compatibility with raw.

Signed-off-by: Pavel Butsykin <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Reviewed-by: John Snow <[email protected]>
Message-id: 20170918124230[email protected]
[mreitz: Added a missing space to a warning]
Signed-off-by: Max Reitz <[email protected]>