nbd/server.c: add coroutine_fn annotations

nbd/server.c: add coroutine_fn annotations

These functions end up calling bdrv_*() implemented as generated_co_wrapper
functions.
In addition, they also happen to be always called in coroutine context,
meaning all callers are coroutine_fn.
This means that the g_c_w function will enter the qemu_in_coroutine()
case and eventually suspend (or in other words call qemu_coroutine_yield()).
Therefore we can mark such functions coroutine_fn too.

Signed-off-by: Emanuele Giuseppe Esposito <[email protected]>
Reviewed-by: Kevin Wolf <[email protected]>
Reviewed-by: Paolo Bonzini <[email protected]>
Reviewed-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Message-Id: <20221128142337[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>