cmd: ubi: don't allow to rename a volume to a name that already exist

This commits add a check on the command ubi rename. This check avoids
to rename a volume to with a name that is already used on another ubi
volume. If two volumes has the same name, then the ubi device can't be
mounted anymore.

Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>