Make preparations for increasing the size of the kernel virtual
address space on the amd64 architecture. The amd64 architecture
requires kernel code and global variables to reside in the highest 2GB
of the 64-bit virtual address space. Thus, KERNBASE cannot change.
However, KERNBASE is sometimes used as the start of the kernel virtual
address space. Henceforth, VM_MIN_KERNEL_ADDRESS should be used
instead. Since KERNBASE and VM_MIN_KERNEL_ADDRESS are still the same
address, there should be no visible effect from this change (yet).