CLOUDSTACK-4648: Fixed Snapshots test cases

Fixed following test cases:
1) test_01_volume_from_snapshot
2) test_03_snapshot_detachedDisk
3) test_07_template_from_snapshot

All three test cases are running successfully on XenServer but test cases (01 and 07) failing on KVM due to a common issue.
Product defect logged for this: https://issues.apache.org/jira/browse/CLOUDSTACK-5097

test_03 is running ok on KVM too, the problem was - disk was not getting detached because it was not unmounted before detaching. Added code to unmount the disk.

Signed-off-by: Girish Shilamkar <girish@clogeny.com>