Description of problem:
When user manually insert a disk block xml in the front of booting disk(installing a os) in guest XML configuration, and user may successfully start the guest, but it seems libvirt hasn't done any check instead of defaultly booting the first disk device in guest XML, however, the first disk probably hasn't any os in it, so libvirt raises a "No bootable device" error in GUI such as virt-viewer and virt-manager, whether libvirt can identify a bootable device by the following xml:
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2' cache='none'/>
      <source file='/var/lib/libvirt/images/rhel6-demo.qcow2'/>
      <target dev='hda' bus='ide'/>
      <address type='drive' controller='0' bus='0' unit='0'/>
    </disk>

    <controller type='ide' index='0'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x1'/>
    </controller>

If it is a libvirt designing solution, user must insert disk xml in the end of booting disk, documents of libvirt need to be enhanced, because of I can't get any relevant information by man page and website of libvirt.  

Version-Release number of selected component (if applicable):
# uname -r
2.6.32-59.1.el6.x86_64

# rpm -q libvirt
libvirt-0.8.1-27.el6.x86_64

# rpm -q qemu-kvm
qemu-kvm-0.12.1.2-2.112.el6.x86_64


How reproducible:
always

Steps to Reproduce:
1. install a guest 
2. shut down the guest
3. editor a disk xml block
4. insert the disk xml in front of booting disk xml
5. start the guest

  
Actual results:
# cat disk.xml 
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='/var/lib/libvirt/images/myqcow2'/>
      <target dev='hdb' bus='virtio'/>
    </disk>

# qemu-img info /var/lib/libvirt/images/myqcow2
image: /var/lib/libvirt/images/myqcow2
file format: qcow2
virtual size: 20M (20971520 bytes)
disk size: 140K
cluster_size: 65536
backing file: /dev/sda1 (actual path: /dev/sda1)

# virsh edit rhel6-demo
Domain rhel6-demo XML configuration edited.

# virsh dumpxml rhel6-demo
<domain type='kvm' id='10'>
  <name>rhel6-demo</name>
  <uuid>55b041fd-9f64-714d-daf2-0ee491b63217</uuid>
  <memory>1048576</memory>
  <currentMemory>1048576</currentMemory>
  <vcpu>1</vcpu>
  <os>
    <type arch='x86_64' machine='rhel6.0.0'>hvm</type>
    <boot dev='hd'/>
  </os>
......
  <devices>
    <emulator>/usr/libexec/qemu-kvm</emulator>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='/var/lib/libvirt/images/myqcow2'/>
      <target dev='hdb' bus='virtio'/>
      <alias name='virtio-disk1'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x06' function='0x0'/>
    </disk>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2' cache='none'/>
      <source file='/var/lib/libvirt/images/rhel6-demo.qcow2'/>
      <target dev='hda' bus='ide'/>
      <alias name='ide0-0-0'/>
      <address type='drive' controller='0' bus='0' unit='0'/>
    </disk>
    <controller type='ide' index='0'>
      <alias name='ide0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x1'/>
    </controller>
......
</domain>

# virsh start rhel6-demo
Domain rhel6-demo started

# virt-viewer rhel6-demo


Expected results:


Additional info:
libvirt passes the following argument to qemu-kvm:
-boot c -drive file=/var/lib/libvirt/images/myqcow2

# ps aux | grep qemu-kvm

qemu      2876  8.5  0.3 1403932 25088 ?       Sl   13:46   0:10 /usr/libexec/qemu-kvm -S -M rhel6.0.0 -enable-kvm -m 1024 -smp 1,sockets=1,cores=1,threads=1 -name rhel6-demo -uuid 55b041fd-9f64-714d-daf2-0ee491b63217 -nodefconfig -nodefaults -chardev socket,id=monitor,path=/var/lib/libvirt/qemu/rhel6-demo.monitor,server,nowait -mon chardev=monitor,mode=control -rtc base=utc -boot c -drive file=/var/lib/libvirt/images/myqcow2,if=none,id=drive-virtio-disk1,boot=on,format=qcow2 -device virtio-blk-pci,bus=pci.0,addr=0x6,drive=drive-virtio-disk1,id=virtio-disk1 -drive file=/var/lib/libvirt/images/rhel6-demo.qcow2,if=none,id=drive-ide0-0-0,format=qcow2,cache=none -device ide-drive,bus=ide.0,unit=0,drive=drive-ide0-0-0,id=ide0-0-0 -netdev tap,fd=26,id=hostnet0 -device rtl8139,netdev=hostnet0,id=net0,mac=52:54:00:ea:24:ef,bus=pci.0,addr=0x3 -chardev tty,id=serial0,path=/dev/ttyS0 -device isa-serial,chardev=serial0 -usb -vnc 127.0.0.1:1 -vga cirrus -device AC97,id=sound0,bus=pci.0,addr=0x4 -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x5