mount: Create binfmt_misc image w/o device suffix

Use name binfmt_misc.img for any mounted binfmt_misc.
Note, that newly created binfmt_misc images won't restore
on old criu.

Iterate over binfmt_misc_list if it's not empty on restore.

Also, because of above, let's change a behaviour of dump and now
we do not create binfmt_misc images if there is no registered
binfmt_misc entries.

v5: New
v6: Use {open,close}_image sequence to determ if image exists
    Iterate over binfmt_misc_list

Suggested-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>