block: bdrv/blk_co_unref() for calls in coroutine context

block: bdrv/blk_co_unref() for calls in coroutine context

These functions must not be called in coroutine context, because they
need write access to the graph.

Cc: qemu-stable@nongnu.org
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20230504115750.54437-4-kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>