vmx: Check serialX.vspc before serialX.fileName

vmx: Check serialX.vspc before serialX.fileName

When using vSPC (Virtual Serial Port Concentrator) in vSphere the actual
address for it is saved in serialX.vspc in which case the
serialX.fileName is most probably something we can't get any useful
information from and we also fail during the parsing rendering any
dumpxml and similar tries unsuccessful.

Instead of parsing the vspc URL with something along the lines of
`virURIParse(vspc ? vspc : fileName)`, which could lead to us reporting
information that is very prune to misuse (the vSPC seemingly has a
protocol on top of the telnet connection; redefining the domain would
change the behaviour; the URL might have a fragment we are not saving;
etc.) or adding more XML knobs to indicate vSPC usage (which we would
not be able to configure; we'd have to properly error out everywhere;
etc.) let's just report dummy serial port that leads to nowhere (i.e.
type="null").

Resolves: https://issues.redhat.com/browse/RHEL-32182
Signed-off-by: Martin Kletzander <mkletzan@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>