iotune: setting an invalid value now reports error

iotune: setting an invalid value now reports error

When trying to set an invalid value into iotune element, standard
behavior was to not report any error, rather to reset all affected
subelements of the iotune element back to 0 which results in ignoring
those particular subelements by XML generator. Patch further
examines the return code of the virXPathULongLong function
and in case of an invalid non-integer value raises an error.
Fixed to preserve consistency with invalid value checking
of other elements.

Resolves https://bugzilla.redhat.com/show_bug.cgi?id=1131811