virDomainControllerDefParseXML: Fix broken error reporting when parsing 'index'

virDomainControllerDefParseXML: Fix broken error reporting when parsing 'index'

The code attempted to report an error if the user added the 'index'
attribute to the 'target' element, but neglected to actually return an
error code.

Fix it by using the VIR_XML_PROP_NONNEGATIVE flag for virXMLPropInt
which refuses user passed negative numbers.

Fixes: 020dd80ecbd
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>