lib: want better Hyper-V overlay page management

[gjcolombo]

#849 takes a very cavalier approach to Hyper-V overlay pages; in particular it does not ensure that a guest physical page's original contents are restored when all overlays are removed. This breaks the semantics described in section 5.2.1 of the Hyper-V TLFS.

The only overlay used in #849 is the hypercall page overlay, and empirically, most guest OSes seem not to care about disabling or relocating this page once they've established it. Nevertheless, Propolis should follow the TLFS's rules for these pages.

Needed for #328.