KVM: selftests: Assign guest page size in sync area early in memslot_perf_test

The guest page size in the synchronization area is needed by all test
cases. So it's reasonable to set it in the unified preparation function
(prepare_vm()).

Signed-off-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Link: https://lore.kernel.org/r/20230118092133.320003-3-gshan@redhat.com


Signed-off-by: Sean Christopherson <seanjc@google.com>