xenapi: through-dev raw-tgz image upload to glance

Upload tgz compressed raw disk images to glance using supported xapi
interfaces.

At the moment, image upload could only be done with dom0 plugins.
These plugins are not supported by Citrix. This change adds an alternate
method, which only uses supported interfaces:

 - Attach the virtual disk to be uploaded to the nova domU as a block
   device.
 - nova reads the block device, and creates a tar.gz stream. The tar.gz
   stream contains only one file, disk.raw, with the block device's
   contents.
 - glance client reads the stream, and updates the image.

In order to use this functionality, in nova.conf, you need to specify:

    xenapi_image_upload_handler =
    nova.virt.xenapi.image.vdi_through_dev.VdiThroughDevStore

DocImpact

implements blueprint xenapi-supported-image-import-export

Change-Id: I7541b8fef3a55939179b6355ed178146c1f8f9d6
Merge "xenapi: through-dev raw-tgz image upload to glance"