chardev: give some context on chardev-add error

chardev: give some context on chardev-add error

Description from Daniel P. Berrangé:
> The original code reported:
>
>  "attempt to add duplicate property 'char2' to object (type 'container')"
>
> Since adding yank support, the current code reports
>
>  "duplicate yank instance"
>
> With this patch applied it now reports:
>
>  "Failed to add chardev 'char2': duplicate yank instance"
>
> This is marginally better, but still not great, not that the original
> error was great either.
>
> It would be nice if we could report
>
>   "chardev with id 'char2' already exists"

Related to:
https://bugzilla.redhat.com/show_bug.cgi?id=1984721

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>