Description of problem:
If a vm is created whose xml description of a disk contains "diskType='volume', the guestfsmod.add_domain fails with the error mentioned in the summary. 
This test has been done using python guestfs module.

If a vm is created whose xml description of a disk contains "diskType='file'", guestfs.add_domain() works fine. 


Version-Release number of selected component (if applicable):
># virt-v2v --version
virt-v2v 1.28.1

# rpm -qa | grep libguestfs
libguestfs-tools-c-1.28.1-1.55.el7.x86_64
python-libguestfs-1.28.1-1.55.el7.x86_64
libguestfs-1.28.1-1.55.el7.x86_64
libguestfs-winsupport-7.2-1.el7.x86_64


How reproducible:
Very

Steps to Reproduce:
Assumption: kvm and libvirt are installed and running. A storage pool called
"default" has been defined whose path points to "/home/vms". 

1. virt-v2v -i ova -of qcow2  -os default -on vsm_vm3 vsm_vmware.ova
  
   Converts ova to a vm with name vsm_vm3. Can see the vm through virsh.
2. Run the script mentioned in the "Additional info". Fails with error mentioned.


Actual results:
#>./test_add_domain.py vsm_vm3
Traceback (most recent call last):
  File "./test_add_domain.py", line 13, in <module>
    g.add_domain(sys.argv[1])
  File "/usr/lib64/python2.7/site-packages/guestfs.py", line 340, in add_domain
    r = libguestfsmod.add_domain (self._o, dom, libvirturi, readonly, iface, live, allowuuid, readonlydisk, cachemode, discard, copyonread)
RuntimeError: libvirt domain has no disks


Expected results:
Expected libguestfsmod.add_domain() to pass instead of throwing a runtime error.

Additional info:
The workaround I found was to:
1.  virt-v2v -i ova -of qcow2 -o local -os /home/vms/ -on vsm_vmware.ova
2. virsh define /home/vms/vsm_vm2.xml
3. This creates a disk of diskType file. Then libguestfs.add_domain() goes through.

Failed VMs xml description of disk:
 <devices>
    <emulator>/usr/libexec/qemu-kvm</emulator>
    <disk type='volume' device='disk'>
      <driver name='qemu' type='qcow2' cache='none'/>
      <source pool='default' volume='vsm_vm3-sda'/>
      <target dev='vda' bus='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/
>
    </disk>


Successful VM's xml description of disk:
  <devices>
    <emulator>/usr/libexec/qemu-kvm</emulator>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2' cache='none'/>
      <source file='/home/vms/vsm_vm2-sda'/>
      <target dev='vda' bus='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/
>
    </disk>


Test script:

#!/usr/bin/env python
import sys
import guestfs

if len(sys.argv) < 2:
    print "Usage: %s <vm name>" % sys.argv[0]
    sys.exit(1)

g = guestfs.GuestFS(python_return_dict=True)

g.add_domain(sys.argv[1])