block: Improve error message when external snapshot can't flush

external_snapshot_action() reports bdrv_flush() failure to its caller
as

    An IO error has occurred

The errno code returned by bdrv_flush() is lost.

Improve this to

    Write to node '<device or node name>' failed: <description of errno>

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240513141703.549874-2-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
1 file changed