Previously it would create a contiguous AVMO manually and pass it to
MM. This uses supervisor pages that quickly run out as they never get
returned and crash the system.
Instead, use allocate_kernel_region as we're only allocating a page so
it will be contiguous and will be returned when destroyed.
A potentially better solution would be to use a pool of transfers to
avoid all the allocations. This just prevents the system from crashing
within ~5 seconds from the continuous hub polling.
We don't need an entirely separate VMObject subclass to influence the
location of the physical pages.
Instead, we simply allocate enough physically contiguous memory first,
and then pass it to the AnonymousVMObject constructor that takes a span
of physical pages.
