dac_nfs_disk: Fix the env restore problem

Fix the qemu conf not restored problem, also update exportfs
option with 'fsid=0' to avoid exportfs problem on fedora.

Signed-off-by: Wayne Sun <gsun@redhat.com>
dac_nfs_disk: Clean up snapshot created imgs

The disk only snapshot img was not cleaned up after reset vm back,
record snapshot disk path after snapshot then clean them up in finally
env restore.

Signed-off-by: Wayne Sun <gsun@redhat.com>
dac_nfs_disk: check file exist before remove

Signed-off-by: Wayne Sun <gsun@redhat.com>
Merge pull request #329 from waynesun09/dac_nfs_fix

dac_nfs_disk: Fix the env restore problem