Good catch - this one was a result of changes made after the initial luks support was added.  More details are in the patch which is posted upstream:

http://www.redhat.com/archives/libvir-list/2016-August/msg00805.html

Adjusted the commit based on review comment see:

http://www.redhat.com/archives/libvir-list/2016-August/msg00867.html

pushed change:

commit b4478c16c02f28d88673709947124c6ea2fb4c7b
Author: John Ferlan <jferlan>
Date:   Wed Aug 17 10:25:43 2016 -0400

    qemu: Fix crash hot plugging luks volume
    
...

    Crash occurs because 'secrets' is being dereferenced in call:
    
            if (qemuDomainSecretSetup(conn, priv, secinfo, disk->info.alias,
                                      VIR_SECRET_USAGE_TYPE_VOLUME, NULL,
                                      &src->encryption->secrets[0]->seclookupdef,
                                      true) < 0)
...

git describe b4478c16c02f28d88673709947124c6ea2fb4c7b
v2.1.0-169-gb4478c1

I can reproduce it with libvirt-2.0.0-5.el7.x86_64
 
Verify version :
libvirt-2.0.0-8.el7.x86_64
qemu-kvm-rhev-2.6.0-24.el7.x86_64

Verify steps :
1. Prepare a swcret as following 
# virsh secret-list 
 UUID                                  Usage
--------------------------------------------------------------------------------
 f981dd17-1234-45bc-88e6-ed1fe20ce9da  volume /var/lib/libvirt/images/luks/luks1.img

2. Prepare a luks pool and create a volume in the pool
# virsh vol-list luks-pool
 Name                 Path                                    
------------------------------------------------------------------------------
 luks                 /var/lib/libvirt/images/luks/luks       
 luks1.img            /var/lib/libvirt/images/luks/luks1.img  

# virsh vol-dumpxml luks luks-pool
<volume type='file'>
  <name>luks</name>
  <key>/var/lib/libvirt/images/luks/luks</key>
  <source>
  </source>
  <capacity unit='bytes'>1074270208</capacity>
  <allocation unit='bytes'>69632</allocation>
  <target>
    <path>/var/lib/libvirt/images/luks/luks</path>
    <format type='raw'/>
    <permissions>
      <mode>0600</mode>
      <owner>107</owner>
      <group>107</group>
      <label>system_u:object_r:svirt_image_t:s0:c300,c387</label>
    </permissions>
    <timestamps>
      <atime>1473648746.592287230</atime>
      <mtime>1473647292.637084255</mtime>
      <ctime>1473648746.152262947</ctime>
    </timestamps>
    <encryption format='luks'>
    </encryption>
  </target>
</volume>

3. define and start a guest 

# virsh list 
 Id    Name                           State
----------------------------------------------------
 2     vm1                            running

using the luks volume as the source of virtual disk
 
# cat file-disk.xml 
<disk type='file' device='disk'>
      <driver name='qemu' type='raw'/>
      <source file='/var/lib/libvirt/images/luks/luks'/>
      <target dev='vdb' bus='virtio'/>
      <encryption format='luks'>
        <secret type='passphrase' uuid='f981dd17-1234-45bc-88e6-ed1fe20ce9da'/>
      </encryption>
    </disk>

attach this disk to guest, it should attach successfully.

# virsh attach-device vm1 file-disk.xml 
Device attached successfully

dumpxml to check 
# virsh dumpxml vm1 | grep disk -A 9
   ...... 
    <disk type='file' device='disk'>
      <driver name='qemu' type='raw'/>
      <source file='/var/lib/libvirt/images/luks/luks'/>
      <backingStore/>
      <target dev='vdb' bus='virtio'/>
      <encryption format='luks'>
        <secret type='passphrase' uuid='f981dd17-1234-45bc-88e6-ed1fe20ce9da'/>
      </encryption>
      <alias name='virtio-disk1'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x06' function='0x0'/>
    </disk>

login guest to check 
#lsblk
it will list the disk 

# virsh detach-device vm1 file-disk.xml 
Device detached successfully

No SIGSEGV now, it can attach successfully. Move to verified.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://rhn.redhat.com/errata/RHSA-2016-2577.html