tests: Remove _NULLABLE in virNetDevExists mock

The @ifname is listed as an ATTRIBUTE_NONNULL(1) parameter, so
checking for _NULLABLE causes a coverity build failure - remove
that and if it's NULL for the test let's fail miserably.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

tests/qemuxml2argvmock.c

@@ -169,7 +169,7 @@ virNetDevSetMAC(const char *ifname G_GNUC_UNUSED,
 int
 virNetDevExists(const char *ifname)
 {
-    return STREQ_NULLABLE(ifname, "mytap0");
+    return STREQ(ifname, "mytap0");
 }