Description
No "Mac address collision" error shows when guests with the same MAC address booted.
When create a new guest from virt-manager,set the fixed MAC address as one which has been used by other guests,error will pop out:
"Mac address collision":"The MAC address '*******' is in use by another virtual machine."

Version:
virt-manager-0.10.0-5.el7.noarch
libvirt-1.1.1-11.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1.Prepare 2 guest xml files,set the same mac address for interface part.
2.use # virsh define guest.xml to create 2 guests.
3.use # virsh start guest to boot 2 guests.
4.The issue also occurs to virtual networks,virtual networks with the same MAC address can be started without error.
# virsh net-list --all
Name                 State      Autostart     Persistent
--------------------------------------------------
bridge-test          active     no            yes
default              active     yes           yes

# virsh net-dumpxml bridge-test
<network>
  <name>bridge-test</name>
  <uuid>2387ed67-23a4-47d4-8227-fa6b0f861760</uuid>
  <forward mode='bridge'/>
  <bridge name='br0' />
  <mac address='52:54:00:3B:78:35'/>
</network>

# virsh net-dumpxml default
<network connections='3'>
  <name>default</name>
  <uuid>196809bf-6249-4800-8152-f9621265378c</uuid>
  <forward mode='nat'/>
  <bridge name='virbr0' stp='on' delay='0' />
  <mac address='52:54:00:3B:78:35'/>
  <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>


Actual results：
Both of the guests can be booted successfully without "Mac address collision" error.

Expected results:
Only 1 guest can be booted,when trying to boot the other guest with the same MAC address,"Mac address collision" error shows.

Additional info:
1.When create a new guest from virt-manager,set the fixed MAC address as one which has been used by other guests,error will pop out:
"Mac address collision":"The MAC address '*******' is in use by another virtual machine."