tests/qtest/migration: Don't use -cpu max for aarch64

The 'max' cpu is not expected to be stable in terms of features across
QEMU versions, so it should not be expected to migrate.

While the tests currently all pass with -cpu max, that is only because
we're not testing across QEMU versions, which is the more common
use-case for migration.

We've recently introduced compatibility tests that use two different
QEMU versions and the tests are now failing for aarch64. The next
patch adds those tests to CI, so we cannot use the 'max' cpu
anymore. Replace it with the 'neoverse-n1', which has a fixed set of
features.

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20240118164951.30350-2-farosas@suse.de
Signed-off-by: Peter Xu <peterx@redhat.com>

tests/qtest/migration-test.c

@@ -820,7 +820,7 @@ static int test_migrate_start(QTestState **from, QTestState **to,
         memory_size = "150M";
         machine_alias = "virt";
         machine_opts = "gic-version=max";
-        arch_opts = g_strdup_printf("-cpu max -kernel %s", bootpath);
+        arch_opts = g_strdup_printf("-cpu neoverse-n1 -kernel %s", bootpath);
         start_address = ARM_TEST_MEM_START;
         end_address = ARM_TEST_MEM_END;
     } else {