qemu-img used to use "image" as ID for all block devices. This means
that e.g. img_convert() ended up with potentially multiple source images
and one target image, all with the same ID. The next patch will catch
this and fail to open the block device.

This patch makes sure that qemu-img uses meaningful unique IDs for the
block devices it uses.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>