dac_nfs_disk: Add test cases for domain vol on netfs pool

Test start domain with netfs pool vol, while netfs server exportfs
with root_squash, no_root_squash and ro options.

The test scenarios including:
1. ro: domain will fail to start with nfs vol.
2. no_root_squash: start domain depends on the nfs file ownership,
   user, group and dynamic_ownership option in qemu.conf.
3. root_squash: start domain depends on the nfs file ownership,
   user and group option in qemu.conf.

Signed-off-by: Wayne Sun <gsun@redhat.com>
dac_nfs_disk: Fix typo

s/successed/succeeded/

Signed-off-by: Wayne Sun <gsun@redhat.com>
dac_nfs_disk: Add test case for disk snapshot with img backing file on nfs

Add test case for bug:
https://bugzilla.redhat.com/show_bug.cgi?id=822015

Signed-off-by: Wayne Sun <gsun@redhat.com>