The s390x virtio bus keeps management information on virtio after the top
of the guest's RAM. We need to be able to tell the guest the size of its
RAM (without virtio stuff), but also be able to trap when the guest accesses
RAM outside of its scope (including virtio stuff).

So we need a variable telling us the size of the virtio stuff, so we can
calculate the highest available RAM address from that.

While at it, also increase the maximum number of virtio pages, so we play
along well with more recent kernels that spawn a ridiculous number of virtio
console adapters.
Signed-off-by: NAlexander Graf <agraf@suse.de>