blockdev-add doesn't know about the device that the backend will be
attached to, this is a legacy -drive concept. Move the remaining checks
that use it to drive_init().

[Fam Zheng <famz@redhat.com> suggested line-wrapping to 80 chars as
required by the coding standard.  I have fixed this.
--Stefan]

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>