selftests: kvm: do only 1 memslot_perf_test run by default

The test takes a long time with the current implementation of
memslots, so cut the run time a bit.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

tools/testing/selftests/kvm/memslot_perf_test.c

@@ -992,7 +992,7 @@ int main(int argc, char *argv[])
 		.tlast = NTESTS - 1,
 		.nslots = -1,
 		.seconds = 5,
-		.runs = 20,
+		.runs = 1,
 	};
 	struct test_result rbestslottime;
 	int tctr;