tests/qtest: s390x: fix operation in a build without any boards or devices

Do the bare minimum to ensure that at least a vanilla
--without-default-devices build works for all targets except i386,
x86_64 and ppc64.  In particular this fixes s390x-softmmu; i386 and
x86_64 have about a dozen failing tests that do not pass -M and therefore
require a default machine type; ppc64 has the same issue, though only
with numa-test.

If we can for now ignore the cases where boards and devices are picked
by hand, drive_del-test however can be fixed easily; almost all tests
check for the virtio-blk or virtio-scsi device that they use, and are
already skipped.  Only one didn't get the memo; plus another one does
not need a machine at all and can be run with -M none.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20240509170044.190795-6-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tests/qtest/drive_del-test.c

@@ -173,7 +173,7 @@ static void test_drive_without_dev(void)
     QTestState *qts;
 
     /* Start with an empty drive */
-    qts = qtest_init("-drive if=none,id=drive0");
+    qts = qtest_init("-drive if=none,id=drive0 -M none");
 
     /* Delete the drive */
     drive_del(qts);
@@ -192,6 +192,11 @@ static void test_after_failed_device_add(void)
     QDict *response;
     QTestState *qts;
 
+    if (!has_device_builtin("virtio-blk")) {
+        g_test_skip("Device virtio-blk is not available");
+        return;
+    }
+
     snprintf(driver, sizeof(driver), "virtio-blk-%s",
              qvirtio_get_dev_type());