virsh: completer: use signed variable for XPathNodeSet errors
virsh: completer: use signed variable for XPathNodeSet errors

Although virXPathNodeSet is unlikely to return -1, we should
check for it properly or not at all.

Signed-off-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>
Reviewed-by: Martin Kletzander <e82f23e74d43760279ebcc505dac4ae70d5627d0@redhat.com>

jardasgit/libvirt,crobinso/libvirt,nertpinx/libvirt,jardasgit/libvirt,nertpinx/libvirt,jardasgit/libvirt,libvirt/libvirt,libvirt/libvirt,crobinso/libvirt,olafhering/libvirt,olafhering/libvirt,jardasgit/libvirt,zippy2/libvirt,jfehlig/libvirt,jfehlig/libvirt,zippy2/libvirt,zippy2/libvirt,crobinso/libvirt,nertpinx/libvirt,zippy2/libvirt,jardasgit/libvirt,nertpinx/libvirt,jfehlig/libvirt,olafhering/libvirt,olafhering/libvirt,crobinso/libvirt,nertpinx/libvirt,libvirt/libvirt,libvirt/libvirt,jfehlig/libvirt