block/coroutines: I/O and "I/O or GS" API

block coroutines functions run in different aiocontext, and are
not protected by the BQL. Therefore are I/O.

On the other side, generated_co_wrapper functions use BDRV_POLL_WHILE,
meaning the caller can either be the main loop or a specific iothread.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Message-Id: <20220303151616.325444-25-eesposit@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>