Adding per-VM snapshot name

Adding per-VM snapshot name as an optional configuration value in < machinemanager >.conf.

If the option is not specified, the machine manager will use its default behaviour, that is:
* KVM and VirtualBox will use the current snapshot, if any;
* VMWare will try to parse the snapshot name from the VM label: this keeps support for older conf files where the VM label was in the form: "path/to/machine.vmx,SnapshotName".