MFC r267311, r267330, r267811, r267884

Turn on interrupt window exiting unconditionally when an ExtINT is being
injected into the guest.

Add helper functions to populate VM exit information for rendezvous and
astpending exits.

Provide APIs to directly get 'lowmem' and 'highmem' size directly.

Expose the amount of resident and wired memory from the guest's vmspace