Description of problem:
Can't remove a rng device when the guest is running sometimes.

Version-Release number of selected component (if applicable):
virt-manager-1.3.2-1.el7.noarch
libvirt-1.3.3-2.el7.x86_64


How reproducible:
50%

Steps to Reproduce:
1. Prepare a shut off guest without rng device.
2. Running the guest and prepare a rng xml.
# cat rng.xml
<rng model="virtio">
  <backend model="random">/dev/random</backend>
</rng>
3.Run "virsh attach-device $guest rng.xml" to attach the rng device to the $guest in Host.
# virsh attach-device $guest rng.xml
Device attached successfully

There will be a rng device in the guest's hardware details.
4.Run "ls /dev|grep hwrng" in the guest.
# ls /dev|grep hwrng
hwrng
5. Run "virsh detach-device $guest rng.xml" to detach the rng device from the $guest in Host.
# virsh detach-device $guest rng.xml
Device detached successfully
6. Run "virsh dumpxml $guest|grep -A4 rng" in Host and check if there is the rng device in hardware details of the guest.


Actual results:
After Step6:
1.#virsh dumpxml $guest|grep -A4 rng
    <rng model='virtio'>
      <backend model='random'>/dev/random</backend>
      <alias name='rng0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x0b' function='0x0'/>
    </rng>
2.There is still a rng device in hardware details of the guest .

Expected results:
1.#virsh dumpxml $guest|grep -A4 rng

2.The rng device in hardware details of the guest is gone.

Additional info:
Please try step 3-6 several times if the bug don't reproduce.