s390-ccw-virtio: allow for systems larger that 7.999TB

KVM does not allow memory regions > KVM_MEM_MAX_NR_PAGES, basically
limiting the memory per slot to 8TB-4k. As memory slots on s390/kvm must
be a multiple of 1MB we need start a new memory region if we cross
8TB-1M.

With that (and optimistic overcommitment in the kernel) I was able to
start a 24TB guest on a 1TB system.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Message-Id: <20171211122146.162430-1-borntraeger@de.ibm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
[CH: 1UL -> 1ULL in KVM_MEM_MAX_NR_PAGES; build fix on 32 bit hosts]
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
