nbd: mark more coroutine_fns

nbd: mark more coroutine_fns

Several coroutine functions in block/nbd.c are not marked as such.  This
patch adds a few more markers; it is not exhaustive, but it focuses
especially on:

- places that wake other coroutines, because aio_co_wake() has very
different semantics inside a coroutine (queuing after yield vs. entering
immediately);

- functions with _co_ in their names, to avoid confusion

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220414175756.671165-3-pbonzini@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@openvz.org>
Reviewed-by: Lukas Straub <lukasstraub2@web.de>
Signed-off-by: Eric Blake <eblake@redhat.com>