DeleteSDKTask.java -> DeleteOldBackupSDKTask.java

For clearer meaning of the class

Signed-off-by: walteryang <walteryang47@gmail.com>
Delete temp snapshot when vm is down

The temp snapshot is created then export backup is performed on a
running vm, because clone is not possible.
As live merge feature is not integrated yet, we have to wait for vm
to power off for temp snapshot deletion

This patch fixes: #4

Signed-off-by: walteryang <walteryang47@gmail.com>