Make instance deletion idempotent (no failure when VM does not exist)

[#83806666]

Signed-off-by: Hua Zhang <zhuadl@cn.ibm.com>
Make disk deletion idempotent (does not error if disk not found)

- Changed stage Skip to require Start to be called first

[#83806666]

Signed-off-by: Hua Zhang <zhuadl@cn.ibm.com>