Description of problem:
The capacity element should be optional when creating a volume with a backing file (by default it should be the same of the base):

# virsh vol-create default /dev/stdin <<EOF
<volume>
  <name>test-base.qcow2</name>
  <capacity unit='bytes'>21474836480</capacity>
  <target>
    <format type='qcow2'/>
  </target>
</volume>
EOF

# virsh vol-create default /dev/stdin <<EOF
<volume>
  <name>test-snapshot1.qcow2</name>
  <target>
    <format type='qcow2'/>
    <permissions>
      <mode>0600</mode>
      <owner>0</owner>
      <group>0</group>
      <label>system_u:object_r:virt_content_t:s0</label>
    </permissions>
  </target>
  <backingStore>
    <path>test-base.qcow2</path>
    <format type='qcow2'/>
  </backingStore>
</volume>
EOF

error: Failed to create vol from /dev/stdin
error: XML error: missing capacity element