util: Add SearchInstance helper

Give an instance name to search if this instance exists under
.capstan/instances.

Signed-off-by: Asias He <asias@cloudius-systems.com>
cmd/stop: Switch to util.SearchInstance

Signed-off-by: Asias He <asias@cloudius-systems.com>
qemu: Support start an stopped instance

It works like below:

    $ capstan run osv.qcow2
    Created instance: i1397047839

    $ capstan stop i1397047839
    Stopped instance: i1397047839

    $ capstan run i1397047839
    Created instance: i1397047839

Note, Instance is not deleted when it is stopped any more. capstan
delete will be added to support delete an instance soon.

Signed-off-by: Asias He <asias@cloudius-systems.com>