tests: qemuargv2xml: hardcode disk auth usage

tests: qemuargv2xml: hardcode disk auth usage

If a qemuargv has iscsi or ceph secrets on the command line, we will
convert that to XML like:

  <auth username='myname'>
    <secret type='iscsi'/>
  </auth>

This is not valid XML, as either a UUID or usage must be specified in
the secret block. It's not clear though how the argv2xml code can do
anything correct here, since XML like this requires a libvirt secret
object to have already been defined.

The current test suite handles this by blanking out any <secret> block
in the XML. This avoids domainschematest failures.

Instead of blanking, let's hardcode a usage= name. This lets us test
the other bits of generated <secret> XML, and is a step towards wiring
up VIR_TEST_REGENERATE_OUTPUT