Description of problem:
the disk type is changed after clone

Version-Release number of selected component (if applicable):
virt-manager-1.4.0-2.el7.noarch

How reproducible:
100%

Steps to Reproduce:
1. Open virt-manager
2. Check the disk type of guest.
    <disk type='volume' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source pool='default' volume='rhel7.2-ftp.qcow2'/>
      <target dev='vda' bus='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x07' function='0x0'/>
    </disk>
3. Clone the guest and check the xml after clone.
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='/var/lib/libvirt/images/rhel7.2-ftp-clone.qcow2'/>
      <target dev='vda' bus='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x07' function='0x0'/>
    </disk>

Actual results:
The disk type is changed from "type='volume'" to "type='file'" after clone.

Expected results:
The disk type should be same as before.

Additional info: