conf: use virXMLPropString for IOThread parsing
conf: use virXMLPropString for IOThread parsing

XPath is good for random search of elements, not for accessing
attributes of one node.

Signed-off-by: Pavel Hrdina <d4772d05997b8abf035041e3b4f4996380ea7e7a@redhat.com>

jfehlig/libvirt,crobinso/libvirt,andreabolognani/libvirt,andreabolognani/libvirt,olafhering/libvirt,andreabolognani/libvirt,zippy2/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,eskultety/libvirt,jfehlig/libvirt,nertpinx/libvirt,zippy2/libvirt,olafhering/libvirt,olafhering/libvirt,jfehlig/libvirt,olafhering/libvirt,nertpinx/libvirt,nertpinx/libvirt,eskultety/libvirt,zippy2/libvirt,jardasgit/libvirt,eskultety/libvirt,libvirt/libvirt,fabianfreyer/libvirt,libvirt/libvirt,crobinso/libvirt,andreabolognani/libvirt,eskultety/libvirt,jardasgit/libvirt,jardasgit/libvirt,libvirt/libvirt,crobinso/libvirt,libvirt/libvirt,zippy2/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,jardasgit/libvirt,crobinso/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,eskultety/libvirt