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>
@yumingfei As only did one time snapshot on all disks with --disk-only, disk_snap_path.append(disk['source']) will got snapshot source file path from xml, so it should work, but your worry is reasonable, I'll update check file path exist first. thx.