ganeti-noded: Fix bug when export didn't succeed for all disks

snap_disks can contain boolean values. They weren't handled correctly.
The error message was “Error while executing backend function: Invalid
object passed to FromDict: expected dict, got <type 'bool'>”.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>