When attempting to convert xml to native qemu and back, domxml-from-native 
reports the following error in trying to convert the native format generated by 
domxml-to-native back to xml:

internal error: missing index/unit/bus parameter in drive 'file=/var/lib/libvirt/images/fedora21_2.qcow2,if=none,id=drive-virtio-disk0,format=qcow2

To reproduce, issue the following commands:

Generate xml for guest created by virt-manager.

# virsh dumpxml fed_21_1 > /tmp/1.xml

$ virsh domxml-to-native qemu-argv /tmp/1.xml > try.txt
$ virsh domxml-from-native qemu-argv try.txt > try.xml

internal error: missing index/unit/bus parameter in drive 'file=/var/lib/libvirt/images/fedora21_2.qcow2,if=none,id=drive-virtio-disk0,format=qcow2

Versions of utilities are shown in detail below: -----------------

Host is fed_21.

$ uname -a
Linux Fedora21 3.18.9-200.fc21.x86_64 #1 SMP Mon Mar 9 15:10:50 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux

virt-manager version 1.1.0

$ virsh --version
virsh version 1.2.9.0

$ virsh --version=long 
Virsh command line tool of libvirt 1.2.9.2
See web site at http://libvirt.org/

Compiled with support for:
 Hypervisors: QEMU/KVM LXC UML Xen LibXL OpenVZ VMWare PHYP VirtualBox ESX Hyper-V Test
  Networking: Remote Network Bridging Interface netcf Nwfilter VirtualPort
   Storage: Dir Disk Filesystem SCSI Multipath iSCSI LVM RBD Gluster
    Miscellaneous: Daemon Nodedev SELinux Secrets Debug DTrace Readline Modular

virsh -c qemu:///system version --daemon
Compiled against library: libvirt 1.2.9
Using library: libvirt 1.2.9
Using API: QEMU 1.2.9
Running hypervisor: QEMU 2.1.3
Running against daemon: 1.2.9

$ qemu --version
QEMU emulator version 2.1.3 (qemu-2.1.3-3.fc21)

Guest is running Fedora 21. It was reated by virt-manager and is fully 
functioning.

xml was created by virsh dumpxml.

Created attachment 1006778 [details]
xml from virsh dumpxml command.

This is the xml from the virsh dumpxml command.

Created attachment 1006780 [details]
native qemu-argv format generated by domxml-to-native command is attached.

This attachment has the native format qemu command generated by dimxml-to-native virsh command.

Just of of curiosity, do you have any use case for this or did you just wanted to try it?

The domxml-from-native command is very limited in what it can do and it doesn't even understand a lot of advanced things libvirt itself puts into a QEMU command line. The main usage of this command is if you previously used QEMU directly and want to switch over to using libvirt. Then the command could help you create an XML which would start QEMU with the same devices you used when running QEMU directly. If it is able to understand the QEMU command line, of course.

That is a good question and my use case is similar. I am trying to test vhost-cuse in DPDK/OVS data plane. I have the host side done and I am now trying to add the vHost device to the guest. I have two examples, one with a sample qemu command line and the other one with edits for the domain xml. I had trouble getting the qemu command to work so I then tried to see if I could edit the additional device info into the xml, but the format was different then my example as documented in OVS perhaps because of a version change in libvirt. Next I googled to see what the recommended way to get the additional device information into the domain xml. Googling, I encountered the following documentation, https://libvirt.org/drvqemu.html#xmlimport which suggested converting the domain to qemu-argv. That is why I tried doing it this way. Before I tried my device and netdev changes to the qemu-argv, I wanted to make sure that the qemu-argv created from the unmolested guest roman xml would work and I ran into the error documented here.
(In reply to Jiri Denemark from comment #4)
> Just of of curiosity, do you have any use case for this or did you just
> wanted to try it?
> 
> The domxml-from-native command is very limited in what it can do and it
> doesn't even understand a lot of advanced things libvirt itself puts into a
> QEMU command line. The main usage of this command is if you previously used
> QEMU directly and want to switch over to using libvirt. Then the command
> could help you create an XML which would start QEMU with the same devices
> you used when running QEMU directly. If it is able to understand the QEMU
> command line, of course.

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Still relevant with libvirt-daemon-1.2.18.1-2.fc23.x86_64

*** This bug has been marked as a duplicate of bug 844845 ***