The public API documents that undefine may be used to transition a
running persistent domain into a transient one.  Many drivers still
do not support this usage, but virsh shouldn't be getting in the
way of those that do support it.

This also drops a redundant conditional; vshCommandOptString
guaranteed that name was non-NULL.

* tools/virsh.c (cmdUndefine): Allow undefine on active domains;
the drivers may still reject it, but it is a valid API usage.
* tests/undefine (error): Fix the test to match.