Use the new virXMLProp helpers and XPath queries to get rid of the old
style of iteration through element children.
Note that in case of def->blockio.logical_block_size,
def->blockio.physical_block_size and def->rotation_rate the wraparound
behaviour of 'virStrToLong_ui' was _not_ forward ported to the new code
as it makes no sense with the attributes.
Signed-off-by: Peter Krempa <pkrempa@redhat.com> Reviewed-by: Michal Privoznik <mprivozn@redhat.com>