virDomainControllerDefParseXML: Reject '-1' for PCI controller target properties

virDomainControllerDefParseXML: Reject '-1' for PCI controller target properties

All of the properties use '-1' as default and the code omits formatting
them when the property is '-1'. Additionally subsequent validation code
rejects all other negative values anyways.

Since we've never formatted '-1' into an XML formatted by libvirt we can
make the parser more strict, as we will never fail to parse existing
on-disk libvirt-owned XMLs.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2121627
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>