selftests: kvm: fix overlapping addresses in memslot_perf_test

vm_create allocates memory and maps it close to GPA.  This memory
is separate from what is allocated in subsequent calls to
vm_userspace_mem_region_add, so it is incorrect to pass the
test memory size to vm_create_default.  Just pass a small
fixed amount of memory which can be used later for page table,
otherwise GPAs are already allocated at MEM_GPA and the
test aborts.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

tools/testing/selftests/kvm/memslot_perf_test.c

@@ -267,7 +267,7 @@ static bool prepare_vm(struct vm_data *data, int nslots, uint64_t *maxslots,
 	data->hva_slots = malloc(sizeof(*data->hva_slots) * data->nslots);
 	TEST_ASSERT(data->hva_slots, "malloc() fail");
 
-	data->vm = vm_create_default(VCPU_ID, mempages, guest_code);
+	data->vm = vm_create_default(VCPU_ID, 1024, guest_code);
 
 	pr_info_v("Adding slots 1..%i, each slot with %"PRIu64" pages + %"PRIu64" extra pages last\n",
 		max_mem_slots - 1, data->pages_per_slot, rempages);