qapi-schema: Update description for NewImageMode

qapi-schema: Update description for NewImageMode

If the NewImageMode is "absolute-paths" but no backing file is available
(e.g., when mirroring a device with an unbacked image), the target image
will not be backed either. This patch updates the documentation in
qapi-schema.json accordingly.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Benoit Canet <benoit@irqsave.net>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>