kvm: Fix memory slot page alignment logic

Memory slots have to be page aligned to get entered into KVM. There
is existing logic that tries to ensure that we pad memory slots that
are not page aligned to the biggest region that would still fit in the
alignment requirements.

Unfortunately, that logic is broken. It tries to calculate the start
offset based on the region size.

Fix up the logic to do the thing it was intended to do and document it
properly in the comment above it.

With this patch applied, I can successfully run an e500 guest with more
than 3GB RAM (at which point RAM starts overlapping subpage memory regions).

Cc: qemu-stable@nongnu.org
Signed-off-by: Alexander Graf <agraf@suse.de>