interface: clean up virInterfaceDefDevFormat

interface: clean up virInterfaceDefDevFormat

This modifies the formatting function of virInterface to be a proper
mirror of the parse function, including the addition of a
"parentIfType" arg so that we can decide whether or not it is
appropriate to emit the elements that are only in toplevel interfaces,
as well as the <link> element (which isn't allowed for bridge
interfaces).

Since the restructuring of the code necessarily changes the order of
some of the elements, some test case data had to be updated.