Mark functions as coroutine_fn when they are only called by other coroutine_fns
and they can suspend.  Because this function operates on a BlockBackend, mark it
GRAPH_UNLOCKED.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <20230601115145.196465-6-pbonzini@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>