Handle missing vm case when fixing missing disk problem

Problem resolver for missing disk first checks if VM is present.

In case of missing VM it does not send unmount to the agent and detach_disk
to the cloud. It only deletes disk reference from DB and sends
delete_disk.

In case of present VM, it tries to unmount disk first. If it does not
succeed that probably means agent is not responding as it tries to
access missing disk. In that case it sends detach_disk to the cloud to update
agent settings.json.

Signed-off-by: Tom Xing <xingzhou@cn.ibm.com>
Vsphere delete_disk is using has_disk? to determine disk presence