Use ve.parseXhtml() and ve.serializeXhtml()

Introduced in VE core in I0fb47f7c91f61.

In IE, this fixes corruption of style and bgcolor attributes.
In other browsers, the only change is that we now use
XMLSerializer rather than .outerHTML to serialize, but as
far as I can tell this isn't materially different. XMLSerializer
correctly uses HTML semantics when given an HTML document.

Bug: 70856
Change-Id: If48555fa4763e763484bbabfa0bc4ded0d0a5c58