x86-64: also handle virtual aliases of Xen image pages

x86-64: also handle virtual aliases of Xen image pages

With the unification of the heaps, the pages freed from the Xen boot
image now can also end up being allocated to a domain, and hence the
respective aliases need handling when such pages get their
cacheability attributes changed.

Rather than establishing multiple mappings with non-WB attributes
(which temporarily still can cause aliasing issues), simply unmap
those pages from the Xen virtual space, and re-map them (to allow re-
establishing of eventual large page mappings) when the cachability
attribute for them gets restored to normal (WB).

Signed-off-by: Jan Beulich <jbeulich@novell.com>