I've posted a couple patches upstream for review, see:

https://www.redhat.com/archives/libvir-list/2017-August/msg00739.html

Testing showed the following results:

# virsh vol-create-as default bz 10M --format qcow2
Vol bz created


# virsh vol-dumpxml bz default | grep format
    <format type='qcow2'/>
# virsh vol-wipe bz default
Vol bz wiped

# virsh vol-dumpxml bz default | grep format
    <format type='raw'/>
# virsh vol-delete bz default
Vol bz deleted

#

NB: For each test pass (without changes and with changes) the vol sizes were:

"After create":

  <capacity unit='bytes'>10485760</capacity>
  <allocation unit='bytes'>200704</allocation>
  <physical unit='bytes'>196616</physical>

"After refresh" (either algorithm, including doing pool refresh):

  <capacity unit='bytes'>200704</capacity>
  <allocation unit='bytes'>200704</allocation>
  <physical unit='bytes'>200704</physical>

So that did not change

Pushed the change upstream:

commit d16f803d780f63b8899df04b624d5acfd6939541
Author: John Ferlan <jferlan>
Date:   Thu Aug 24 18:19:14 2017 -0400

    storage: Use virStorageBackendRefreshVolTargetUpdate after wipeVol
    
...
    
    Rather than using refreshVol which essentially only updates the
    allocation, capacity, and permissions for the volume, but not
    the format which does get updated in a pool refresh - let's use
    the same helper that pool refresh uses in order to update the
    volume target.


$ git describe d16f803d780f63b8899df04b624d5acfd6939541
v3.7.0-rc1-13-gd16f803
$

Test env components:
libvirt-3.7.0-2.el7.x86_64
qemu-kvm-rhev-2.9.0-16.el7_4.6.x86_64
kernel-3.10.0-709.el7.x86_64

Test scenarios:
Scenario-1: Wipe a volume without backing file
1. Prepare a volume named '1.qcow2' in 'default' pool, and display the dumpxml file info of the volume
# virsh pool-list --all |grep default
 default              active     yes   

# qemu-img info /var/lib/libvirt/images/1.qcow2 --backing-chain
image: /var/lib/libvirt/images/1.qcow2
file format: qcow2
virtual size: 1.0G (1073741824 bytes)
disk size: 196K
cluster_size: 65536
Format specific information:
    compat: 1.1
    lazy refcounts: false
    refcount bits: 16
    corrupt: false

# virsh vol-list default |grep 1.qcow2
 1.qcow2              /var/lib/libvirt/images/1.qcow2  

# virsh vol-dumpxml 1.qcow2 default
  ...
  <capacity unit='bytes'>1073741824</capacity>
  <allocation unit='bytes'>200704</allocation>
  <physical unit='bytes'>196624</physical>
  ...
    <path>/var/lib/libvirt/images/1.qcow2</path>
    <format type='qcow2'/>
  ...

2. Wipe the volume and then check the dumpxml file info of the volume
# virsh vol-wipe 1.qcow2 default
Vol 1.qcow2 wiped

# virsh vol-dumpxml 1.qcow2 default
  ...
  <capacity unit='bytes'>200704</capacity>
  <allocation unit='bytes'>200704</allocation>
  <physical unit='bytes'>200704</physical>
  ...
    <path>/var/lib/libvirt/images/1.qcow2</path>
    <format type='raw'/>
  ...

3. Check the dumpxml file info of the volume after refreshing the pool
# virsh pool-refresh default
Pool default refreshed

# virsh vol-dumpxml 1.qcow2 default
  ...
  <capacity unit='bytes'>200704</capacity>
  <allocation unit='bytes'>200704</allocation>
  <physical unit='bytes'>200704</physical>
  ...
    <path>/var/lib/libvirt/images/1.qcow2</path>
    <format type='raw'/>
  ...

Scenario-2: Wipe a volume with backing file
1. Prepare a volume named '2.qcow2' in 'default' pool with backing file named '1.qcow2', and display the dumpxml file info of the volume
# virsh pool-list --all |grep default
 default              active     yes  

# qemu-img info /var/lib/libvirt/images/2.qcow2 --backing-chain
image: /var/lib/libvirt/images/2.qcow2
file format: qcow2
virtual size: 1.0G (1073741824 bytes)
disk size: 196K
cluster_size: 65536
backing file: 1.qcow2 (actual path: /var/lib/libvirt/images/1.qcow2)
Format specific information:
    compat: 1.1
    lazy refcounts: false
    refcount bits: 16
    corrupt: false

image: /var/lib/libvirt/images/1.qcow2
file format: qcow2
virtual size: 1.0G (1073741824 bytes)
disk size: 196K
cluster_size: 65536
Format specific information:
    compat: 1.1
    lazy refcounts: false
    refcount bits: 16
    corrupt: false

# virsh vol-list default |grep 2.qcow2
 2.qcow2              /var/lib/libvirt/images/2.qcow2    

# virsh vol-dumpxml 2.qcow2 default
  ...
  <capacity unit='bytes'>1073741824</capacity>
  <allocation unit='bytes'>200704</allocation>
  <physical unit='bytes'>196624</physical>
  <target>
    <path>/var/lib/libvirt/images/2.qcow2</path>
    <format type='qcow2'/>
  </target>
  ...
  <backingStore>
    <path>/var/lib/libvirt/images/1.qcow2</path>
    <format type='qcow2'/>
  </backingStore>
  ...

2. Wipe the volume and then check the dumpxml file info of the volume
# virsh vol-wipe 2.qcow2 default
Vol 2.qcow2 wiped

# virsh vol-dumpxml 2.qcow2 default
  ...
  <capacity unit='bytes'>200704</capacity>
  <allocation unit='bytes'>200704</allocation>
  <physical unit='bytes'>200704</physical>
  <target>
    <path>/var/lib/libvirt/images/2.qcow2</path>
    <format type='raw'/>
  </target>
  <backingStore>
    <path>/var/lib/libvirt/images/1.qcow2</path>
    <format type='qcow2'/>
  </backingStore>
  ...

3. Check the dumpxml file info of the volume after refreshing the pool
Note: Backing file is not supported for file format 'raw'.
# virsh pool-refresh default
Pool default refreshed

# virsh vol-dumpxml 2.qcow2 default
  ...
  <capacity unit='bytes'>200704</capacity>
  <allocation unit='bytes'>200704</allocation>
  <physical unit='bytes'>200704</physical>
  <target>
    <path>/var/lib/libvirt/images/2.qcow2</path>
    <format type='raw'/>
  </target>
  ...

All the results are as expected, move this bug to be 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://access.redhat.com/errata/RHEA-2018:0704