Description of problem:
It shouldn't be permitted to change the uuid of a nwfilter

Version-Release number of selected component (if applicable):
libvirt-1.1.1-27.el7.x86_64
qemu-kvm-rhev-1.5.3-53.el7.x86_64
kernel-3.10.0-110.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1. Dump a nwfilter
[root@localhost ~]# virsh nwfilter-dumpxml mynwfilter
<filter name='mynwfilter' chain='root'>
  <uuid>71ef822b-4220-4752-b1aa-3653d40254a1</uuid>
  <rule action='drop' direction='out' priority='-650'>
    <tcp dstportstart='22'/>
  </rule>
</filter>

2. Change its uuid to new, like "71ef822b-4220-4752-b1aa-3653d4025999" 
[root@localhost ~]# virsh nwfilter-edit mynwfilter
Network filter mynwfilter XML configuration edited.

[root@localhost ~]# virsh nwfilter-dumpxml mynwfilter
<filter name='mynwfilter' chain='root'>
  <uuid>71ef822b-4220-4752-b1aa-3653d4025999</uuid>
  <rule action='drop' direction='out' priority='-650'>
    <tcp dstportstart='22'/>
  </rule>
</filter>


Actual results:
As shown above. Its uuid was changed.

Expected results:
I think libvirt should prevent those actions and report some warning/error message like virtual network/domain.

[root@localhost ~]# virsh net-dumpxml default
<network>
  <name>default</name>
  <uuid>02ddfd5e-8b4b-48b2-ad16-e0876e76f4da</uuid>
  <forward mode='nat'>
    <nat>
      <port start='1024' end='65535'/>
    </nat>
  </forward>
  <bridge name='virbr0' stp='on' delay='0' />
  <mac address='52:54:00:58:33:1b'/>
  <ip address='192.168.122.1' netmask='255.255.255.0'>
    <dhcp>
      <range start='192.168.122.2' end='192.168.122.254' />
    </dhcp>
  </ip>
</network>

[root@localhost ~]# virsh net-edit default
error: operation failed: network 'default' already exists with uuid 02ddfd5e-8b4b-48b2-ad16-e0876e76f4da
Failed. Try again? [y,n,f,?]:


[root@localhost ~]# virsh dumpxml r7| grep uuid -a5
<domain type='kvm' id='6'>
  <name>r7</name>
  <uuid>6495a404-dc7b-4b8a-8ea3-29c6896d0849</uuid>
  <memory unit='KiB'>1048576</memory>
  <currentMemory unit='KiB'>1048576</currentMemory>
  <vcpu placement='static'>1</vcpu>
  <resource>
    <partition>/machine</partition>

[root@localhost ~]# virsh edit r7
error: operation failed: domain 'r7' already exists with uuid 6495a404-dc7b-4b8a-8ea3-29c6896d0849
Failed. Try again? [y,n,f,?]:


Additional info:
I have a candidate patch for this issue, could you accept it?

diff --git a/src/conf/nwfilter_conf.c b/src/conf/nwfilter_conf.c
index 5599443..ca5696a 100644
--- a/src/conf/nwfilter_conf.c
+++ b/src/conf/nwfilter_conf.c
@@ -2978,6 +2978,17 @@ virNWFilterObjAssignDef(virNWFilterObjListPtr nwfilters,
             return NULL;
         }
         virNWFilterObjUnlock(nwfilter);
+    } else {
+           nwfilter = virNWFilterObjFindByName(nwfilters, def->name);
+           if (nwfilter) {
+              char uuidstr[VIR_UUID_STRING_BUFLEN];
+              virUUIDFormat(nwfilter->def->uuid, uuidstr);
+              virReportError(VIR_ERR_OPERATION_FAILED,
+                             _("filter '%s' already exists with uuid %s"),
+                             nwfilter->def->name, uuidstr);
+              virNWFilterObjUnlock(nwfilter);
+              return NULL;
+            }    
     }

     if (virNWFilterDefLoopDetect(nwfilters, def) < 0) {


[root@amd-5600-4-1 x86_64]# virsh nwfilter-dumpxml clean-traffic-9 
<filter name='clean-traffic-9' chain='root'>
  <uuid>415c9b7e-efd1-4099-a9a4-d9589d8d85e4</uuid>
  <filterref filter='no-mac-spoofing'/>
  <filterref filter='no-ip-spoofing'/>
  <rule action='accept' direction='out' priority='-600'>
    <mac protocolid='ipv4'/>
  </rule>
  <filterref filter='allow-incoming-ipv4'/>
  <filterref filter='no-arp-spoofing'/>
  <rule action='accept' direction='inout' priority='-500'>
    <mac protocolid='arp'/>
  </rule>
  <filterref filter='no-other-l2-traffic'/>
  <filterref filter='qemu-announce-self'/>
</filter>

[root@amd-5600-4-1 x86_64]# virsh nwfilter-edit clean-traffic-9
error: operation failed: filter 'clean-traffic-9' already exists with uuid 415c9b7e-efd1-4099-a9a4-d9589d8d85e4
Failed. Try again? [y,n,f,?]: