KVM: selftests: Convert iterations to int in dirty_log_perf_test

In order to add an iteration -1 to indicate that the memory population
phase has not yet completed, convert the interations counters to ints.

No functional change intended.

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

Signed-off-by: Ben Gardon <bgardon@google.com>
Message-Id: <20210112214253.463999-4-bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>