Description of problem:
If change the guest memory during installation on Xen PV, after installation, guest can not start and get the error:
Error starting domain: POST operation failed: xend_post: error from xen daemon: (xend.err 'Error creating domain: Kernel image does not exist: /var/lib/xen/virtinst-vmlinuz.eWCmvP')

Version-Release number of selected component (if applicable):
virt-manager-0.6.1-16.el5
libvirt-0.8.2.24.el5
python-virtinst-0.400.3-13.el5
kernel-xen-2.6.18-300.el5
xen-3.0.3-135.el5

# uname -a
Linux localhost.localdomain 2.6.18-300.el5xen #1 SMP Thu Dec 1 14:08:23 EST 2011 x86_64 x86_64 x86_64 GNU/Linux

How reproducible:
always

Steps to Reproduce:
1). Install a new PV guest by virt-manager.
2). After choose the packages which you want to install, click next to install the guest.
3). Before finish the installation, change the memory to 1023 through the virt-manager.
4). After installation, start the guest.
5). Check the xml file of guest, the boot config "<bootloader>/usr/bin/pygrub</bootloader>" do not saved.
<domain type='xen'>
  <name>5.7pv</name>
  <uuid>10287014-9ab2-d727-2404-5725c87ac002</uuid>
  <memory>1048576</memory>
  <currentMemory>1047552</currentMemory>
  <vcpu>3</vcpu>
  <os>
    <type arch='x86_64' machine='xenpv'>linux</type>
    <kernel>/var/lib/xen/virtinst-vmlinuz.eWCmvP</kernel>
    <initrd>/var/lib/xen/virtinst-initrd.img.vBX-ni</initrd>
    <cmdline>method=http://download.englab.nay.redhat.com/pub/rhel/released/RHEL-5-Server/U7/x86_64/os/</cmdline>
  </os>
  <clock offset='utc'/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>destroy</on_reboot>
  <on_crash>destroy</on_crash>
  <devices>
    <disk type='file' device='disk'>
      <driver name='tap' type='aio'/>
      <source file='/var/lib/xen/images/5.7pv.img'/>
      <target dev='xvda' bus='xen'/>
    </disk>
    <interface type='bridge'>
      <mac address='00:16:3e:31:61:42'/>
      <source bridge='virbr0'/>
      <script path='vif-bridge'/>
      <target dev='vif1.0'/>
    </interface>
    <console type='pty'>
      <target port='0'/>
    </console>
    <input type='mouse' bus='xen'/>
    <graphics type='vnc' port='-1' autoport='yes' keymap='en-us'/>
  </devices>
</domain>

  
Actual results:
Guest can't start and get the error:
Error starting domain: POST operation failed: xend_post: error from xen daemon: (xend.err 'Error creating domain: Kernel image does not exist: /var/lib/xen/virtinst-vmlinuz.eWCmvP')

Expected results:
The boot config "<bootloader>/usr/bin/pygrub</bootloader>" should be saved in xml and the guest can start successfully.

Additional info:
If don't change the memory during installation, the guest can start successfully after installation.
If add "<bootloader>/usr/bin/pygrub</bootloader>" in the guest xml file manually, the guest can start successfully, after start, the xml of guest will be changed to:
#virsh dumpxml 5.7pv
<domain type='xen' id='3'>
  <name>5.7pv</name>
  <uuid>10287014-9ab2-d727-2404-5725c87ac002</uuid>
  <memory>1048576</memory>
  <currentMemory>1047552</currentMemory>
  <vcpu>3</vcpu>
  <bootloader>/usr/bin/pygrub</bootloader>
  <os>
    <type>linux</type>
    <kernel>/var/lib/xen/boot_kernel.49vXVT</kernel>
    <initrd>/var/lib/xen/boot_ramdisk.eoRGC_</initrd>
    <cmdline>ro root=/dev/VolGroup00/LogVol00 rhgb quiet</cmdline>
  </os>
...