KVM: selftests: Fix population stage in dirty_log_perf_test

Currently the population stage in the dirty_log_perf_test does nothing
as the per-vCPU iteration counters are not initialized and the loop does
not wait for each vCPU. Remedy those errors.

Reviewed-by: Jacob Xu <jacobhxu@google.com>
Reviewed-by: Makarand Sonare <makarandsonare@google.com>

Signed-off-by: Ben Gardon <bgardon@google.com>
Message-Id: <20210112214253.463999-5-bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>