blkverify: Add locking for request_fn

blkverify: Add locking for request_fn

This is either bdrv_co_preadv() or bdrv_co_pwritev() which both need to
have the graph locked. Annotate the function pointer accordingly and add
locking to its callers.

This shouldn't actually have resulted in a bug because the graph lock is
already held by blkverify_co_prwv(), which waits for the coroutines to
terminate. Annotate with GRAPH_RDLOCK as well to make this clearer.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20231027155333.420094-19-kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>