block: Use bdrv_co_* instead of synchronous versions in coroutines

If we're already in a coroutine, there is no reason to use the synchronous
version of block layer functions when a coroutine one exists. This makes
bdrv_read/write/flush use bdrv_co_* when used inside a coroutine.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>