Currently on dump we generate too many image files, effectively
all the stuff from the GLOB set is created. The thing is that
sometimes some of created images can be empty (just contain the
magic number at the head). Thos images are useless and just
waste the space.

When applied after the "empty images" set, this introduces the
lazy images -- when we call open_image() the actual file is
only created (and the magic number is written into it) when the
very first object goes into it.

For example for the simplest test we have, then static/env00
one, the created image files are

   core-7290.img
   creds-7290.img
   fdinfo-2.img
   fs-7290.img
   ids-7290.img
   inventory.img
   mm-7290.img
   pagemap-7290.img
   pages-1.img
   pstree.img
   reg-files.img
   sigacts-7290.img
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>