Close leaked file handles in container config, CRIU stats, and playbook read

Add missing defer Close() calls in four locations:

- libpod/container.go: specFromState() opens the container config file
  but never closes it after reading, leaking one fd per call.

- libpod/container_internal_common.go: checkpoint() and restore() each
  open the bundle directory for CRIU statistics but never close it,
  leaking one fd per checkpoint/restore operation.

- pkg/machine/shim/host.go: Init() opens the playbook file but never
  closes it after ReadAll, leaking one fd per machine init.

Signed-off-by: Sebastien Tardif <sebtardif@ncf.ca>