trusty:

host~$ virsh domif-setlink lp1718568-trusty vnet4 down
Device updated successfully

ubuntu@lp1718568-trusty:~$ dpkg -l | grep isc-dhcp
ii  isc-dhcp-client                  4.2.4-7ubuntu12.12                         amd64        ISC DHCP client
ii  isc-dhcp-common                  4.2.4-7ubuntu12.12                         amd64        common files used by all the isc-dhcp* packages
ubuntu@lp1718568-trusty:~$ sudo ip l set down dev eth1
ubuntu@lp1718568-trusty:~$ sudo ip l show eth1
3: eth1: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 1000
    link/ether 52:54:00:1f:1a:c6 brd ff:ff:ff:ff:ff:ff

ubuntu@lp1718568-trusty:~$ sudo dhclient -v -6 eth1
Internet Systems Consortium DHCP Client 4.2.4
Copyright 2004-2012 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/

no link-local IPv6 address for eth1


with -updates pkg:

ubuntu@lp1718568-trusty:~$ dpkg -l | grep isc-dhcp
ii  isc-dhcp-client                  4.2.4-7ubuntu12.13                         amd64        ISC DHCP client
ii  isc-dhcp-common                  4.2.4-7ubuntu12.13                         amd64        common files used by all the isc-dhcp* packages
ubuntu@lp1718568-trusty:~$ sudo ip l set down dev eth1
ubuntu@lp1718568-trusty:~$ sudo ip l show eth1
3: eth1: <BROADCAST,MULTICAST> mtu 1500 qdisc pfifo_fast state DOWN mode DEFAULT group default qlen 1000
    link/ether 52:54:00:1f:1a:c6 brd ff:ff:ff:ff:ff:ff

ubuntu@lp1718568-trusty:~$ sudo dhclient -v -6 eth1
Internet Systems Consortium DHCP Client 4.2.4
Copyright 2004-2012 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/

(blocks)

host$ virsh domif-setlink lp1718568-trusty vnet4 up
Device updated successfully

Listening on Socket/eth1
Sending on   Socket/eth1
PRC: Soliciting for leases (INIT).
XMT: Forming Solicit, 0 ms elapsed.
XMT:  X-- IA_NA 00:1f:1a:c6
XMT:  | X-- Request renew in  +3600
XMT:  | X-- Request rebind in +5400
XMT: Solicit on eth1, interval 1010ms.
RCV: Advertise message on eth1 from fe80::5054:ff:fec9:897c.
RCV:  X-- IA_NA 00:1f:1a:c6
RCV:  | X-- starts 1520536242
RCV:  | X-- t1 - renew  +3600
RCV:  | X-- t2 - rebind +7200
RCV:  | X-- [Options]
RCV:  | | X-- IAADDR 2001:db8::93
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 00:1f:1a:c6
XMT:  | X-- Requested renew  +3600
XMT:  | X-- Requested rebind +5400
XMT:  | | X-- IAADDR 2001:db8::93
XMT:  | | | X-- Preferred lifetime +7200
XMT:  | | | X-- Max lifetime +7500
XMT:  V IA_NA appended.
XMT: Request on eth1, interval 1040ms.
RCV: Reply message on eth1 from fe80::5054:ff:fec9:897c.
RCV:  X-- IA_NA 00:1f:1a:c6
RCV:  | X-- starts 1520536243
RCV:  | X-- t1 - renew  +3600
RCV:  | X-- t2 - rebind +7200
RCV:  | X-- [Options]
RCV:  | | X-- IAADDR 2001:db8::93
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.