hmp: info spice: Show string channel name

Useful for debugging.

https://bugzilla.redhat.com/show_bug.cgi?id=822418
Signed-off-by: Cole Robinson <crobinso@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
