Description of problem:

The 'reboot' operation for xen fencing does not work.  This has two causes:

(a) libvirt does not have a guaranteed immediate synchronous reboot option.  The
only "reboot" operation it has is worthless, because it is equivalent to a
soft-shutdown / init 6 / ACPI soft-powerdown event.  Supposedly, this could be
implemented with the xen 3.0.3 APIs, but it does not appear to be in libvirt, at
least not according to the documentation here:

  http://libvirt.org/html/libvirt-libvirt.html#virDomainReboot

(b) xend unlinks the temporary images set up by pygrub, making the
virDomainGetXMLDesc() information about the kernel and initrd.


A solution to (b) above is to kill the entire <os> tag.  Here's an example XML
desc from virDomainGetXMLDesc:

<domain type='xen' id='10'>
  <name>kanderso-xen-02</name>
  <uuid>165b5ef7e37713750409e70cf9cbd43c</uuid>
  <bootloader>/usr/bin/pygrub</bootloader>
  <os>
    <type>linux</type>
    <kernel>/var/lib/xen/vmlinuz.CRKPf-</kernel>
    <initrd>/var/lib/xen/initrd.kcwg-N</initrd>
    <cmdline>ro root=/dev/VolGroup00/LogVol00 console=xvc0 rhgb quiet</cmdline>
  </os>
  <memory>393216</memory>
  <vcpu>1</vcpu>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>restart</on_crash>
  <devices>
    <interface type='bridge'>
      <source bridge='xenbr1'/>
      <mac address='00:16:3e:37:02:d9'/>
      <script path='vif-bridge'/>
    </interface>
    <disk type='file' device='disk'>
      <driver name='tap' type='aio'/>
      <source file='/tmp/futon2.dsk'/>
      <target dev='xvda'/>
    </disk>
    <console tty='/dev/pts/4'/>
  </devices>
</domain>


All we need to do is blank-out the <os> tag and all children, and it will fall
back to pygrub.