block: switch bdrv_read()/bdrv_write() to coroutines

block: switch bdrv_read()/bdrv_write() to coroutines

The bdrv_read()/bdrv_write() functions call .bdrv_read()/.bdrv_write().
They should go through bdrv_co_do_readv() and bdrv_co_do_writev()
instead in order to unify request processing code across sync, aio, and
coroutine interfaces.  This is also an important step towards removing
BlockDriverState .bdrv_read()/.bdrv_write() in the future.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>