block: replace bdrv_states iteration with bdrv_next()

block: replace bdrv_states iteration with bdrv_next()

The bdrv_states list is a static variable in block.c.

bdrv_drain_all() and bdrv_flush_all() use this variable to iterate over
all drives.

The next patch will move bdrv_drain_all() and bdrv_flush_all() out of
block.c so it's necessary to switch to the public bdrv_next() interface.

Reviewed-by: Alberto Garcia <[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>