s390x: initialize the physical allocator

We now have the range of free memory, let's initialize the physical
allocator. It is now possible to use use malloc and memalign, based on the
early-ops. alloc_pages() cannot be used yet (as it has to be initialized
via free_pages() - e.g. via setup_vm() - first)

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20180213162321.20522-8-david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>