Add API for starting/stopping drives. This new API will enable
GVolumeMonitor and GVfs implementations to add support for the
following features

 1. Powering down external hard disk enclosures / drives

 2. Starting/stopping multi-disk devices (such as RAID/btrfs/ZFS)

 3. Connecting/disconnecting iSCSI devices

 4. Reacting to the user pressing e.g. the "remove drive" button on
    a IBM/Lenovo Ultrabay: http://www.thinkwiki.org/wiki/Ultrabay

See the bug for the corresponding GVfs and Nautilus changes.