dac_nfs_save_restore: Add test cases for save and restore domain with nfs file

For save domain to nfs file, 2 scenarios need be concerned:
  1. the file did not exist, root_squash will fail
  2. the file exist, similar to dac_nfs_disk test scenarios

In this commit, restore from file is following behind succeeded domain
saving, so the rule for restore follows as save.

Signed-off-by: Wayne Sun <gsun@redhat.com>