Description of problem:
Unable to stably view libvirt host lists via avahi with virt-manager

Version-Release number of selected component (if applicable):
libvirt-2.0.0-10.el7_3.2.x86_64
virt-manager-1.4.0-2.el7.noarch

How reproducible:
Above 90%

Steps to Reproduce:
1.Make sure firewall allows mDNS
# system-config-firewall (use #firewall-config instead if FirewallD is active)
Tick mDNS in pop-up firewall config windows

2.Config a remote server in the same subnet:
2.1 Edit /etc/libvirt/libvirtd.conf
mdns_adv = 1
mdns_name = "Virtualization Host first-Demo"
2.2 Edit /etc/nsswitch.conf
hosts: hostname
2.3 Restart service
# systemctl restart avahi-daemon
# systemctl restart libvirtd

3.Config another server, called "Virtualization Host second-Demo".

4.Check above virtual hostname on Client host
4.1 In virt-manager, click "File"-> "Add Connection",
Select hypervisor as "QEMU/KVM", Tick "Connect to remote host", Method as "SSH", Username as "root"
4.2 Check "hostname" dropdown list

5.Stop and start virt-manager again, check the "hostname" dropdown list to see different results.

6.Using avahi-discover instead:
#avahi-discover
In the popup window, both "Virtualization Host first-Demo" and "Virtualization Host second-Demo" are always shown under the item "Virtual Machine Manager", clearly and stably.


Actual results:
Step 4&5: In virt-manager, cannot show libvirt hosts stably:
Sometimes can show both, sometimes show none(the dropdown button disabled), also sometimes show only one.

Expected results:
In virt-manager, should show all libvirt hosts in "hostname" dropdown list every time. Just as step6 avahi-discover displayed.