vbox, vmw: Support start an stopped instance

It works like below:

   $ capstan run osv.vmdk
   Created instance: i1397047839

   $ capstan stop i1397047839
   Stopped instance: i1397047839

   $ capstan run i1397047839
   Created instance: i1397047839

Signed-off-by: Asias He <asias@cloudius-systems.com>
vmw: Delete ConfigFile as well

vmxRun deleteVM will not delete the file not created by vmxRun. Delete
it ourself.

Signed-off-by: Asias He <asias@cloudius-systems.com>
vbox: Delete ConfigFile as well

VBoxManage  unregistervm --delete will not delete the file not created
by VBoxManage. Delete it ourself.

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