file: Find descs by ID only (v2)

Actually all file-s we dump have unique IDs, regardless of
their types. This fact will be used to reduce complexity
of the SCM code -- instead of keeping TYPE:ID pair it'll
save only the ID.

Siad that -- we will need the way to lookup desc by ID only.

v2: Older images had fifo-s and tty-s having matching IDs
    with respective reg-file entries

Reviewed-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>