Description of problem:
client get ipv6 address which do not in the dnsmasq dhcp range.

Version-Release number of selected component (if applicable):
# rpm -q libvirt dnsmasq
libvirt-3.7.0-2.el7.x86_64
dnsmasq-2.76-4.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1. As libvirt use dnsmasq as a dhcp server, start a guest connected to a network 'net', the guest can get  ipv6 address which in the defined range.
# virsh net-dumpxml net
<network connections='1' ipv6='yes'>
  <name>net</name>
  <uuid>ca9ea2a9-eed4-431d-b281-90458b7d83ea</uuid>
  <forward mode='nat'>
    <nat>
      <port start='1024' end='65535'/>
    </nat>
  </forward>
  <bridge name='virbr1' stp='on' delay='0'/>
  <mac address='52:54:00:7a:9e:b1'/>
  <ip address='192.168.100.1' netmask='255.255.255.0'>
  </ip>
  <ip family='ipv6' address='2001:db8:ca2:2::1' prefix='64'>
    <dhcp>
      <range start='2001:db8:ca2:2:5::100' end='2001:db8:ca2:2:5::1ff'/>
    </dhcp>
  </ip>
</network>

# ifconfig eth0
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet6 2001:db8:ca2:2:5::1e7  prefixlen 64  scopeid 0x0<global>
        inet6 fe80::5054:ff:fe7b:5191  prefixlen 64  scopeid 0x20<link>
        ether 52:54:00:7b:51:91  txqueuelen 1000  (Ethernet)

# cat /var/lib/libvirt/dnsmasq/net.conf
##WARNING:  THIS IS AN AUTO-GENERATED FILE. CHANGES TO IT ARE LIKELY TO BE
##OVERWRITTEN AND LOST.  Changes to this configuration should be made using:
##    virsh net-edit net
## or other application using the libvirt API.
##
## dnsmasq conf file created by libvirt
strict-order
pid-file=/var/run/libvirt/network/net.pid
except-interface=lo
bind-dynamic
interface=virbr1
dhcp-range=2001:db8:ca2:2:5::100,2001:db8:ca2:2:5::1ff,64
dhcp-lease-max=256
dhcp-hostsfile=/var/lib/libvirt/dnsmasq/net.hostsfile
addn-hosts=/var/lib/libvirt/dnsmasq/net.addnhosts
enable-ra

2.  destroy the 'dhcp server' and 'client' for step 3 to make the changes.
# virsh destroy yal; virsh net-destroy net
Domain yal destroyed

Network net destroyed


3. change the ipv6 ip-dhcp-range for the network, then start the network and vm

# virsh net-start net; virsh net-dumpxml net
Network net started

<network ipv6='yes'>
  <name>net</name>
  <uuid>ca9ea2a9-eed4-431d-b281-90458b7d83ea</uuid>
  <forward mode='nat'>
    <nat>
      <port start='1024' end='65535'/>
    </nat>
  </forward>
  <bridge name='virbr1' stp='on' delay='0'/>
  <mac address='52:54:00:7a:9e:b1'/>
  <ip address='192.168.100.1' netmask='255.255.255.0'>
  </ip>
  <ip family='ipv6' address='2001:db8:ca2:2::1' prefix='64'>
    <dhcp>
      <range start='2001:db8:ca2:2:7::200' end='2001:db8:ca2:2:7::2ff'/>
    </dhcp>
  </ip>
</network>

# virsh start yal
Domain yal started

# cat /var/lib/libvirt/dnsmasq/net.conf
##WARNING:  THIS IS AN AUTO-GENERATED FILE. CHANGES TO IT ARE LIKELY TO BE
##OVERWRITTEN AND LOST.  Changes to this configuration should be made using:
##    virsh net-edit net
## or other application using the libvirt API.
##
## dnsmasq conf file created by libvirt
strict-order
pid-file=/var/run/libvirt/network/net.pid
except-interface=lo
bind-dynamic
interface=virbr1
dhcp-range=2001:db8:ca2:2:7::200,2001:db8:ca2:2:7::2ff,64
dhcp-lease-max=256
dhcp-hostsfile=/var/lib/libvirt/dnsmasq/net.hostsfile
addn-hosts=/var/lib/libvirt/dnsmasq/net.addnhosts
enable-ra


4. log in guest and check the ipv6 address do not change, but it is not in the current range
# ifconfig -a
eth0: flags=4098<BROADCAST,MULTICAST>  mtu 1500
        ether 52:54:00:7b:51:91  txqueuelen 1000  (Ethernet)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0
....
# dhclient -6 
# ifconfig 
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet6 2001:db8:ca2:2:5::1e7  prefixlen 64  scopeid 0x0<global>
        inet6 fe80::5054:ff:fe7b:5191  prefixlen 64  scopeid 0x20<link>
        ether 52:54:00:7b:51:91  txqueuelen 1000  (Ethernet)

Actual results:
client get ipv6 address which do not in the current dnsmasq dhcp range.

Expected results:
the guest should get a ipv6 address in the range of  "<range start='2001:db8:ca2:2:7::200' end='2001:db8:ca2:2:7::2ff'/>"

Additional info:
It seems the guest or host will remember what he has got or allocated last time until the lease time even the range changed.