restore: Merge self vmas alltogether

When getting hint for restorer blob we scan the list of
vmas in criu address space. Since these vmas are only
required to find holes, we can merge vmas alltogether
and make restorer_get_vma_hint's work easier.

Reduces list of items in small zdtm tests from 71 to 9.

Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>