Description of problem:
virsh command 'domxml-from-native qemu-argv' can not parse the virtio block device correctly.

Version-Release number of selected component (if applicable):
libvirt upstream

How reproducible:

cat /tmp/guest.args
/usr/local/bin/qemu-system-x86_64 -machine accel=kvm -name test -machine pc-0.15,accel=kvm,usb=off -m 512 -realtime mlock=off -smp 2,sockets=2,cores=1,threads=1 -nographic -no-user-config -nodefaults  -chardev socket,id=charmonitor,path=/var/lib/libvirt/qemu/test.monitor,server,nowait -mon chardev=charmonitor,id=monitor,mode=control -rtc base=utc -no-shutdown -drive file=/ovirt-storage/rhel6.img,if=none,id=drive-virtio-disk0,format=raw,index=0 -device virtio-blk-pci,scsi=off,bus=pci.0,addr=0x5,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1 -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x6

virsh domxml-from-native qemu-argv /tmp/guest.args
<domain type='kvm' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>
  <name>test</name>
  <uuid>0a0baaf5-2a21-404c-99c4-378b3cf61da7</uuid>
  <memory unit='KiB'>524288</memory>
  <currentMemory unit='KiB'>524288</currentMemory>
  <vcpu placement='static'>2</vcpu>
  <os>
    <type arch='x86_64' machine='pc-0.15'>hvm</type>
  </os>
  <features>
    <acpi/>
    <pae/>
  </features>
  <cpu>
    <topology sockets='2' cores='1' threads='1'/>
  </cpu>
  <clock offset='utc'/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>destroy</on_crash>
  <devices>
    <emulator>/usr/local/bin/qemu-system-x86_64</emulator>
    <disk type='file' device='disk'>
      <driver name='qemu' type='raw'/>
      <source file='/ovirt-storage/rhel6.img'/>
      <target dev='hda' bus='ide'/>
      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
    </disk>
    <controller type='ide' index='0'/>
    <controller type='usb' index='0'/>
    <controller type='pci' index='0' model='pci-root'/>
    <memballoon model='virtio'/>
  </devices>
  <qemu:commandline>
    <qemu:arg value='-realtime'/>
    <qemu:arg value='mlock=off'/>
    <qemu:arg value='-no-user-config'/>
    <qemu:arg value='-nodefaults'/>
    <qemu:arg value='-device'/>
    <qemu:arg value='virtio-blk-pci,scsi=off,bus=pci.0,addr=0x5,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1'/>
    <qemu:arg value='-device'/>
    <qemu:arg value='virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x6'/>
  </qemu:commandline>
</domain>


cat /tmp/guest.args.1
/usr/bin/qemu-kvm -machine pc-0.14,accel=kvm  -m 1024 -smp 1,sockets=1,cores=1,threads=1 -drive file=/ovirt-storage/rhel6.img,if=virtio,id=drive-virtio-disk0,format=qcow2 -device virtio-blk-pci,scsi=off,bus=pci.0,addr=0x6,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1

/usr/bin/qemu-kvm -machine pc-0.14,accel=kvm  -m 1024 -smp 1,sockets=1,cores=1,threads=1 -drive file=/ovirt-storage/rhel6.img,if=virtio,id=drive-virtio-disk0,format=qcow2 -device virtio-blk-pci,scsi=off,bus=pci.0,addr=0x6,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1
qemu-kvm: -drive file=/ovirt-storage/rhel6.img,if=virtio,id=drive-virtio-disk0,format=qcow2: Property 'virtio-blk-pci.drive' can't take value 'drive-virtio-disk0', it's in use

QEMU report error.

But, domxml-from-native work normally:

virsh domxml-from-native qemu-argv /tmp/guest.args.1
<domain type='kvm' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>
  <name>unnamed</name>
  <uuid>f752995a-4946-4beb-a9f1-951d055a8884</uuid>
  <memory unit='KiB'>1048576</memory>
  <currentMemory unit='KiB'>1048576</currentMemory>
  <vcpu placement='static'>1</vcpu>
  <os>
    <type arch='x86_64' machine='pc-0.14'>hvm</type>
  </os>
  <features>
    <acpi/>
    <pae/>
  </features>
  <cpu>
    <topology sockets='1' cores='1' threads='1'/>
  </cpu>
  <clock offset='utc'/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>destroy</on_crash>
  <devices>
    <emulator>/usr/bin/qemu-kvm</emulator>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='/ovirt-storage/rhel6.img'/>
      <target dev='vda' bus='virtio'/>
    </disk>
    <controller type='usb' index='0'/>
    <controller type='pci' index='0' model='pci-root'/>
    <input type='mouse' bus='ps2'/>
    <graphics type='sdl'/>
    <video>
      <model type='cirrus' vram='9216' heads='1'/>
    </video>
    <memballoon model='virtio'/>
  </devices>
  <qemu:commandline>
    <qemu:arg value='-device'/>
    <qemu:arg value='virtio-blk-pci,scsi=off,bus=pci.0,addr=0x6,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1'/>
  </qemu:commandline>
</domain>


Actual results:
For /tmp/guest.args, the bus of the disk parsed as IDE.
For /tmp/guest.args.1, it parse the disk as virtio block.

Expected results:
For /tmp/guest.args, the bus of the disk should be parsed as a virtio disk.
For /tmp/guest.args.1, it should report error.