libvirt: Save device_path in connection_info when booting from volume

If you boot an instance from a volume and later terminate it, the
libvirt volume driver disconnect_volume method does not have the
'device_path' key in connection_info['data'].  However, if you
attach a volume to an existing instance and then detach it,
the disconnect_volume method would have the 'device_path' key in
connection_info['data'].  Having the 'device_path' key would be
useful for some volume drivers to determine the device path of the
volume.  This patch saves the 'device_path' in connection_info['data']
when _create_domain_and_network is called, so it could be later used.

Change-Id: I8ebb5f3c2e7a81b11d776f8c0a15f3491ed273be
Closes-Bug: #1291007
Merge "libvirt: Save device_path in connection_info when booting from volume"