block: Make the block accounting functions operate on BlockAcctStats

block: Make the block accounting functions operate on BlockAcctStats

This is the next step for decoupling block accounting functions from
BlockDriverState.
In a future commit the BlockAcctStats structure will be moved from
BlockDriverState to the device models structures.

Note that bdrv_get_stats was introduced so device models can retrieve the
BlockAcctStats structure of a BlockDriverState without being aware of it's
layout.
This function should go away when BlockAcctStats will be embedded in the device
models structures.

CC: Kevin Wolf <[email protected]>
CC: Stefan Hajnoczi <[email protected]>
CC: Keith Busch <[email protected]>
CC: Anthony Liguori <[email protected]>
CC: "Michael S. Tsirkin" <[email protected]>
CC: Paolo Bonzini <[email protected]>
CC: Eric Blake <[email protected]>
CC: Peter Maydell <[email protected]>
CC: Michael Tokarev <[email protected]>
CC: John Snow <[email protected]>
CC: Markus Armbruster <[email protected]>
CC: Alexander Graf <[email protected]>
CC: Max Reitz <[email protected]>
Signed-off-by: Benoît Canet <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>