migration-test: dirtylimit checks for x86_64 arch before

So no need to assert we are in x86_64.
Once there, refactor the function to remove useless variables.

Reviewed-by: Peter Xu <peterx@redhat.com>
Message-ID: <20230608224943.3877-11-quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

tests/qtest/migration-test.c

@@ -2651,10 +2651,7 @@ static int64_t get_limit_rate(QTestState *who)
 static QTestState *dirtylimit_start_vm(void)
 {
     QTestState *vm = NULL;
-    g_autofree gchar *cmd = NULL;
-    const char *arch = qtest_get_arch();
-
-    assert((strcmp(arch, "x86_64") == 0));
+    g_autofree gchar *
     cmd = g_strdup_printf("-accel kvm,dirty-ring-size=4096 "
                           "-name dirtylimit-test,debug-threads=on "
                           "-m 150M -smp 1 "