qemu-img: Improve error messages for failed bdrv_open
Output the error message string of the bdrv_open return code. Also set a non-empty device name for the images because the unknown feature error message includes it. Signed-off-by: NKevin Wolf <kwolf@redhat.com> Reviewed-by: NAnthony Liguori <aliguori@us.ibm.com>
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