Perform dumping but with preliminary iterations. Each
time an iteration ends the ->more callback is called.
The callback's return value is
	- positive -- one more iteration starts
	- zero     -- final dump is performed and call exits
	- negative -- dump is aborted, the value is returned
back from criu_dump_iters

Inside callback one may (well, should) call criu_set_
function to alter the details of next iterations. In
particluar, then prev and next images directories should
be changed.

The @pi argument is an opaque value that caller may
use to request pre-dump statistics (not yet implemented).
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>