Do this by adding a helper function to get the persistent domain config. This
should be useful for other functions that may eventually want to alter
the persistent domain config (attach/detach device). Also make similar changes
to the test drivers setvcpus command.

A caveat is that the function will return the running config for a transient
domain, rather than error. This simplifies callers, as long as they use
other methods to ensure the guest is persistent.