KVM: selftests: allow two iterations of dirty_log_perf_test

Even though one iteration is not enough for the dirty log performance
test (due to the cost of building page tables, zeroing memory etc.)
two is okay and it is the default.  Without this patch,
"./dirty_log_perf_test" without any further arguments fails.

Cc: Ben Gardon <bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>