KVM: PPC: Avoid referencing userspace memory region in memslot updates

For PPC HV, get the number of pages directly from the new memslot instead
of computing the same from the userspace memory region, and explicitly
check for !DELETE instead of inferring the same when toggling mmio_update.
The motivation for these changes is to avoid referencing the @mem param
so that it can be dropped in a future commit.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Message-Id: <1e97fb5198be25f98ef82e63a8d770c682264cc9.1638817639.git.maciej.szmigiero@oracle.com>