qemu-img: Use BlockBackend as far as possible

qemu-img: Use BlockBackend as far as possible

Although qemu-img already creates BlockBackends, it does not do accesses
to the images through them. This patch converts all of the bdrv_* calls
for which this is currently possible to blk_* calls. Most of the
remaining calls will probably stay bdrv_* calls because they really do
operate on the BDS level instead of the BB level.

Signed-off-by: Max Reitz <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Message-id: 1423162705[email protected]
Signed-off-by: Stefan Hajnoczi <[email protected]>