Description of problem:

trying to generate snapshot command (from vdsm) failed on the following libvirt error:

Libvirt Error:
==============

Traceback (most recent call last):
  File "/usr/share/vdsm/libvirtvm.py", line 1226, in snapshot
    libvirt.VIR_DOMAIN_SNAPSHOT_CREATE_DISK_ONLY)
  File "/usr/share/vdsm/libvirtvm.py", line 458, in f
    ret = attr(*args, **kwargs)
  File "/usr/share/vdsm/libvirtconnection.py", line 79, in wrapper
    ret = f(*args, **kwargs)
  File "/usr/lib64/python2.7/site-packages/libvirt.py", line 1310, in snapshotCreateXML
    if ret is None:raise libvirtError('virDomainSnapshotCreateXML() failed', dom=self)
libvirtError: unsupported configuration: source for disk 'vdb' is not a regular file; refusing to generate external snapshot name

Snapshot Request
================

<domainsnapshot>
  <disks>
    <disk name="vda" snapshot="external">
      <source file="/rhev/data-center/2b027b34-cb8f-4315-a544-65a7ae888178/48939185-3631-42dd-b39c-f59175a3df38/images/f75a5ee2-54dc-45b0-a412-853b1b240dea/1017b8f7-8ba4-4e90-9f65-d57cca0fd8f6"/>
    </disk>
  </disks>
</domainsnapshot>

Domain Definition
=================

<?xml version="1.0" encoding="utf-8"?>
<domain type="kvm">
        <name>Fed4</name>
        <uuid>6bc41e20-348b-4ccd-b4cd-21bd243bbe83</uuid>
        <memory>524288</memory>
        <currentMemory>524288</currentMemory>
        <vcpu>1</vcpu>
        <devices>
                <disk device="disk" type="block">
                        <source dev="/rhev/data-center/2b027b34-cb8f-4315-a544-65a7ae888178/48939185-3631-42dd-b39c-f59175a3df38/images/4363969c-ecee-4a6d-ad91-7267860e0061/612fccf4-dac7-4d39-9665-fbfd42136739"/>
                        <target bus="virtio" dev="vda"/>
                        <serial>6d-ad91-7267860e0061</serial>
                        <driver cache="none" error_policy="stop" io="native" name="qemu" type="qcow2"/>
                </disk>
                <disk device="disk" type="block">
                        <source dev="/rhev/data-center/2b027b34-cb8f-4315-a544-65a7ae888178/48939185-3631-42dd-b39c-f59175a3df38/images/f75a5ee2-54dc-45b0-a412-853b1b240dea/fd3a4083-45b9-45a5-ba67-98f3cf49fd4d"/>
                        <target bus="virtio" dev="vdb"/>
                        <serial>b0-a412-853b1b240dea</serial>
                        <driver cache="none" error_policy="stop" io="native" name="qemu" type="raw"/>
                </disk>
                <disk device="cdrom" type="file">
                        <target bus="ide" dev="hdc"/>
                </disk>
                <controller index="0" ports="16" type="virtio-serial"/>
                <channel type="unix">
                        <target name="com.redhat.rhevm.vdsm" type="virtio"/>
                        <source mode="bind" path="/var/lib/libvirt/qemu/channels/Fed4.com.redhat.rhevm.vdsm"/>
                </channel>
                <memballoon model="none"/>
                <interface type="bridge">
                        <mac address="00:1a:4a:16:01:5d"/>
                        <model type="virtio"/>
                        <source bridge="engine"/>
                </interface>
                <input bus="ps2" type="mouse"/>
                <video>
                        <model heads="1" type="qxl" vram="65536"/>
                </video>
                <channel type="spicevmc">
                        <target name="com.redhat.spice.0" type="virtio"/>
                </channel>
                <graphics autoport="yes" keymap="en-us" passwd="*****" passwdValidTo="1970-01-01T00:00:01" port="-1" tlsPort="-1" type="spice">
                        <channel mode="secure" name="main"/>
                        <channel mode="secure" name="inputs"/>
                        <listen network="vdsm-engine" type="network"/>
                </graphics>
        </devices>
        <os>
                <type arch="x86_64" machine="pc-0.14">hvm</type>
                <boot dev="hd"/>
                <smbios mode="sysinfo"/>
        </os>
        <sysinfo type="smbios">
                <system>
                        <entry name="manufacturer">Red Hat</entry>
                        <entry name="product">RHEV Hypervisor</entry>
                        <entry name="version">16-1</entry>
                        <entry name="serial">44454C4C-5700-1035-8056-B8C04F4A4E31_00:26:B9:67:14:7B</entry>
                        <entry name="uuid">6bc41e20-348b-4ccd-b4cd-21bd243bbe83</entry>
                </system>
        </sysinfo>
        <clock adjustment="0" offset="variable">
                <timer name="rtc" tickpolicy="catchup"/>
        </clock>
        <features>
                <acpi/>
        </features>
        <cpu match="exact">
                <model>Opteron_G1</model>
                <topology cores="1" sockets="1" threads="1"/>
        </cpu>
</domain>


libvirt git: commit 98b942c6f51f0b383f9438fe1ec678b6e8c4984e
vdsm git: commit 67800f45d6c776a9964327429ff6ddc8feea353a

attached snapshot script as well.