ploop replace: initial implementation

This is a bare bone implementation of ploop replace, a command to
replace a non-top-level ploop image by another one. Note that images
should be identical.

To do:
1. implement version with DiskDescriptor.xml argument
2. implement ploop_replace_image() for ext. users
3. document in man page
4. do more testing

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Revert "BUGFIX Redesing the read DiskDescriptor"

This reverts commit 3249c55d4bcf7469942fdf0ca5e55bba154e1a28.
Patch was committed by mistake; to be reimplemented.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>