Add disk config value to xenstore

disk_config has been introduced by the extension 'DiskConfig'( [[http://docs.rackspace.com/openstack-extensions/compute/rax-dcf/content/rax-dcf.pdf]] ) .This extension exposes api to set and retrieve the disk_config values ( either 'MANUAL" or "AUTO' ) .As a user one wants to access the value of disk_config from inside of instance but currently there is no way to get the value of disk_config from inside of instance.
To get any metadata/userdata of the instance a config drive ( [[http://docs.openstack.org/trunk/openstack-compute/admin/content/config-drive.html]] ) can be attached to the instance during boot-up.So disk_drive information can also be included in the config drive.