ploop replace: implement form with disk descriptor

This is an addition to the previous patch, adding the higher-level
replace command that operates with DiskDescriptor.xml. An image
to be replaced is specified by either UUID or LEVEL.

This also adds a new API function, ploop_replace_image().

TODO: man page

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