libvirt: fix a simple bug in virDomainSetMemoryFlags()

This patch fix a simple bug in virDomainSetMemoryFlags function.
The patch sent before lacks the consideration of the case
where the driver doesn't support virDomainSetMemoryFlags API.
Signed-off-by: NTaku Izumi <izumi.taku@jp.fujitsu.com>

src/libvirt.c

@@ -2902,6 +2902,8 @@ virDomainSetMemoryFlags(virDomainPtr domain, unsigned long memory,
         return ret;
     }
 
+    virLibConnError(VIR_ERR_NO_SUPPORT, __FUNCTION__);
+
 error:
     virDispatchError(domain->conn);
     return -1;