block/qapi: always report full_backing_filename

block/qapi: always report full_backing_filename

Always report full_backing_filename, even if it's the same as
backing_filename. In the next patch, full_backing_filename may be
omitted if it cannot be generated instead of allowing e.g. drive_query
to abort if it runs into this scenario.

The presence or absence of the "full" field becomes useful information.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 1450122916-4706-3-git-send-email-jsnow@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>