qemuxmlactivetest: Prepare for proper active/inactive -> active/inactive testing

qemuxmlactivetest: Prepare for proper active/inactive -> active/inactive testing

Currently the xml->xml testing we have in qemuxml2xmltest covers only 3
of the 4 possibilities:

By invocation:
  active -> active;
  inactive -> inactive;

by unintentionally:
  active -> inactive  (for configs which don't set an 'id' as the
                       formatter assumes it's inactive)

To do it better introduce proper active -> inactive/active testing into
qemuxmlactivetest. It's chosen such as we only really parse an XML as
live when restoring a status XML. To give users possibility to avoid
constructing a full status XML add a simpler variant. As of such it will
be used only for configs where we specifically cared about parsing live
data.

To ensure that the formatter doesn't decide that a config is inactive
because it doesn't have an ID we fill in a domain ID if it was not
present in the source.

In this patch the tests are not yet added.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>