KVM: PPC: Make PAM a define

On PowerPC it's very normal to not support all of the physical RAM in real mode.
To check if we're matching on the shared page or not, we need to know the limits
so we can restrain ourselves to that range.

So let's make it a define instead of open-coding it. And while at it, let's also
increase it.

Signed-off-by: Alexander Graf <agraf@suse.de>

v2 -> v3:

  - RMO -> PAM (non-magic page)
Signed-off-by: Avi Kivity <avi@redhat.com>