tests/megasas: Make test independent of global_qtest

The test uses memwrite() and thus relies on global_qtest. Let's replace it
with qtest_memwrite(), so that we are independent from global_qtest here.

Message-Id: <20190409085245.31548-3-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

tests/megasas-test.c

@@ -66,7 +66,7 @@ static void megasas_pd_get_info_fuzz(void *obj, void *data, QGuestAllocator *all
     context[7] = cpu_to_le32(0);
 
     context_pa = guest_alloc(alloc, sizeof(context));
-    memwrite(context_pa, context, sizeof(context));
+    qtest_memwrite(dev->bus->qts, context_pa, context, sizeof(context));
     qpci_io_writel(dev, bar, 0x40, context_pa);
 }