InbandEventStream should be an element, slice

[bdstark]

CommonAttributesAndElements.InbandEventStream has an XML tag to be an attribute, but should be an element:

InbandEventStream         *DescriptorType       `xml:"inbandEventStream,attr"`

should be

InbandEventStream         *DescriptorType       `xml:"InbandEventStream,omitempty"`

otherwise it will never be unmarshaled.

I think is is valid to have multiple InbandEventStream elements, which would need more than just this fix.

InbandEventStream         []DescriptorType      `xml:"InbandEventStream,omitempty"`