Description of problem:
Guest with udp backend rng device should fail to start when no random data resource or when to connect a unknown host ip

Version-Release number of selected component (if applicable):
libvirt-2.0.0-9.el7.x86_64
Qemu-kvm-rhev-2.6.0-25.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1. Prepare a guest has following rng device in xml:
    <rng model='virtio'>
     <backend model='egd' type='udp'>
       <source mode='connect' host='localhost' service='1234'/>
     </backend>
    </rng>

2. Do not give any random data resource for it, and try to start the guest
The guest is started unexpectedly.

3. Destroy the guest, edit the guest's xml with following rng device, ip=1.2.3.4 is a non-exist host:
     <rng model='virtio'>
     <backend model='egd' type='udp'>
       <source mode='connect' host='1.2.3.4' service='1234'/>
     </backend>
    </rng>

4. Try to start the guest again
The guest is started unexpectedly, too.

5. Start a normal guest without rng device, and try to attach the rng device by xml as step 1 descripted.
Device attached successfully.

6. Start a normal guest without rng device, and try to attach the rng device by xml as step 3 descripted.
Failed to attach the rng device, but error is not correct:
error: internal error: unable to execute QEMU command 'chardev-add': Failed to bind socket: Cannot assign requested address

Actual results:
As above.
1.In step 2&4, guest with udp backend rng device can be started when no random data resource is given or when to connect a unknown host ip(1.2.3.4).
2.In step 5, a udp rng device can be attached even no random data resource is given.
3.In step 6, a udp rng device with a non-exist host ip failed to be attached with a uncorrect error, seems like a syntax error, because I used 'connect' source mode but not a 'bind' source mode.

Expected results:
1.In step 2, should fail to start with "connection refused" error;
  In step 4, should fail to start with "time out" error.
2.In step 5, a udp rng device should fail to be attached with "connection refused" error.
3.In step 6, failed error should be correct such as "time out" error.

Additional info:
1.The result tested by tcp backend are as follows, more reasonable.
    <rng model='virtio'>
     <backend model='egd' type='tcp'>
       <source mode='connect' host='1.2.3.4'(or localhost) service='1024'/>
       <protocol type='raw'/>
     </backend>
    </rng>
(1)Start guest without random data resource(host=localhost):
error: internal error: qemu unexpectedly closed the monitor: 2016-09-19T11:33:56.330256Z qemu-kvm: -chardev socket,id=charrng0,host=localhost,port=1024: Failed to connect socket: Connection refused
(2)Start guest with non-exit host ip(host='1.2.3.4'):
error: internal error: qemu unexpectedly closed the monitor: 2016-09-19T11:09:45.723691Z qemu-kvm: -chardev socket,id=charrng0,host=1.2.3.4,port=1024: Failed to connect socket: Connection timed out
(3)Hotplug the tcp backend rng device without random data resource(host=localhost):
error: internal error: unable to execute QEMU command 'chardev-add': Failed to connect socket: Connection refused
(4)Hotplug the tcp backend rng device with non-exit host ip(host='1.2.3.4'):
error: internal error: unable to execute QEMU command 'chardev-add': Failed to connect socket: Connection timed out