pycriu:images: treat pagemap.img as a special one

pagemap.img is the special one, as it starts with an entry
of pagemap_head type and is followed by entries of
pagemap_entry type. Currently we treat pagemap as if
pagemap_head is a payload and pagemap_entry's are
an extra, which is wrong.

Signed-off-by: Ruslan Kuprieiev <kupruser@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>