xenial:

ubuntu@lp1718568-xenial:~$ dpkg -l | grep isc-dhcp
ii  isc-dhcp-client                  4.3.3-5ubuntu12.9                          amd64        DHCP client for automatically obtaining an IP address
ii  isc-dhcp-common                  4.3.3-5ubuntu12.9                          amd64        common files used by all of the isc-dhcp packages

host$ virsh domif-setlink lp1718568-xenial vnet6 down
Device updated successfully

ubuntu@lp1718568-xenial:~$ sudo dhclient -v -6 ens7
Internet Systems Consortium DHCP Client 4.3.3
Copyright 2004-2015 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/

no link-local IPv6 address for ens7

If you think you have received this message due to a bug rather
than a configuration issue please read the section on submitting
bugs on either our web page at www.isc.org or in the README file
before submitting a bug.  These pages explain the proper
process and the information we find helpful for debugging..

exiting.



with -proposed pkg:

ubuntu@lp1718568-xenial:~$ dpkg -l | grep isc-dhcp
ii  isc-dhcp-client                  4.3.3-5ubuntu12.10                         amd64        DHCP client for automatically obtaining an IP address
ii  isc-dhcp-common                  4.3.3-5ubuntu12.10                         amd64        common files used by all of the isc-dhcp packages
ubuntu@lp1718568-xenial:~$ sudo ip l set down dev ens7
ubuntu@lp1718568-xenial:~$ sudo ip l show dev ens7
3: ens7: <BROADCAST,MULTICAST> mtu 1500 qdisc pfifo_fast state DOWN mode DEFAULT group default qlen 1000
    link/ether 52:54:00:f3:99:a8 brd ff:ff:ff:ff:ff:ff

ubuntu@lp1718568-xenial:~$ sudo dhclient -v -6 ens7
Internet Systems Consortium DHCP Client 4.3.3
Copyright 2004-2015 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/

(blocks)

$ virsh domif-setlink lp1718568-xenial vnet6 up
Device updated successfully

Listening on Socket/ens7
Sending on   Socket/ens7
Created duid \000\001\000\001\"4D\337RT\000\363\231\250.
PRC: Soliciting for leases (INIT).
XMT: Forming Solicit, 0 ms elapsed.
XMT:  X-- IA_NA 00:f3:99:a8
XMT:  | X-- Request renew in  +3600
XMT:  | X-- Request rebind in +5400
XMT: Solicit on ens7, interval 1040ms.
RCV: Advertise message on ens7 from fe80::5054:ff:fec9:897c.
RCV:  X-- IA_NA 00:f3:99:a8
RCV:  | X-- starts 1520535647
RCV:  | X-- t1 - renew  +3600
RCV:  | X-- t2 - rebind +7200
RCV:  | X-- [Options]
RCV:  | | X-- IAADDR 2001:db8::98
RCV:  | | | X-- Preferred lifetime 604800.
RCV:  | | | X-- Max lifetime 2592000.
RCV:  X-- Server ID: 00:01:00:01:22:2c:61:12:52:54:00:c9:89:7c
RCV:  Advertisement recorded.
PRC: Selecting best advertised lease.
PRC: Considering best lease.
PRC:  X-- Initial candidate 00:01:00:01:22:2c:61:12:52:54:00:c9:89:7c (s: 155, p: 0).
XMT: Forming Request, 0 ms elapsed.
XMT:  X-- IA_NA 00:f3:99:a8
XMT:  | X-- Requested renew  +3600
XMT:  | X-- Requested rebind +5400
XMT:  | | X-- IAADDR 2001:db8::98
XMT:  | | | X-- Preferred lifetime +7200
XMT:  | | | X-- Max lifetime +7500
XMT:  V IA_NA appended.
XMT: Request on ens7, interval 990ms.
RCV: Reply message on ens7 from fe80::5054:ff:fec9:897c.
RCV:  X-- IA_NA 00:f3:99:a8
RCV:  | X-- starts 1520535648
RCV:  | X-- t1 - renew  +3600
RCV:  | X-- t2 - rebind +7200
RCV:  | X-- [Options]
RCV:  | | X-- IAADDR 2001:db8::98
RCV:  | | | X-- Preferred lifetime 604800.
RCV:  | | | X-- Max lifetime 2592000.
RCV:  X-- Server ID: 00:01:00:01:22:2c:61:12:52:54:00:c9:89:7c
PRC: Bound to lease 00:01:00:01:22:2c:61:12:52:54:00:c9:89:7c.