virsh_setvcpus: Update test

Update the config/test to follow more "modern" practices, especially
with respect to _domain vs. _vm_ref variables in cfg file. Found instances
where _vm_ref was being used instead of _options - fixed that too. Added
tests for add and remove when the guest is on (hotswap). This code has
undergone quite a bit of change and this test was always trying to remove
a vcpu which has it's issues.

Adjust test flow to use try/finally for the test conditions. Rather than
relying on /etc/libvirt/qemu/%s.xml for running/defined domain output,
use the dumpxml() function with and without --inactive based on whether
the test is adjusting the "--config" or otherwise.  Adjust the usage of
temporary xml files to use the tmp directory.  Use the more commonly
used new_from_inactive_dumpxml() to save/restore the original configuration.
Finally rather than relying on the generated xml format to re.search
to find the current value, grab that from the XML.  The issue with the
current code is that a new property "placement=<string>" was added and
the result was the re.search would never succeed the way it was written.

Use TestNAError for remote tests when not configured properly and also
for the libvirt/qemu error string check. The hotswap/remove cpus is the
most difficult option