DO NOT MERGE: Hack-a-test to verify gpc invalidation+refresh

DO NOT MERGE: Hack-a-test to verify gpc invalidation+refresh

Add a VM-wide gfn=>pfn cache and a fake MSR to let userspace control the
cache.  On writes, reflect the value of the MSR into the backing page of
a gfn=>pfn cache so that userspace can detect if a value was written to
the wrong page, i.e. to a stale mapping.

Spin up 16 vCPUs (arbitrary) to use/refresh the cache, and another thread
to trigger mmu_notifier events and memslot updates.

Co-authored-by: David Woodhouse <dwmw@amazon.co.uk>
Not-signed-off-by: Sean Christopherson <seanjc@google.com>
Not-signed-off-by: David Woodhouse <dwmw@amazon.co.uk>