Could reproduce this bug on the following components of rh5.6:
xen-3.0.3-120.el5
libvirt-0.8.2-15.el5
kernel-xen-2.6.18-238.el5

1 create floppy img:
# qemu-img create -f raw /tmp/floppy.img 1440k
Formatting '/tmp/floppy.img', fmt=raw, size=1440 kB

# mkfs.msdos -s 1 /tmp/floppy.img 
mkfs.msdos 2.11 (12 Mar 2005)

2. define a guest with the following xml:
# cat tmp 
<domain type='xen'>
  <name>dom_test11</name>
  <memory>1048576</memory>
  <currentMemory>1048576</currentMemory>
  <vcpu>1</vcpu>
  <os>
    <type>hvm</type>
    <loader>/usr/lib/xen/boot/hvmloader</loader>
    <boot dev='cdrom'/>
  </os>
  <features>
    <acpi/>
    <apic/>
    <pae/>
  </features>
  <clock offset='utc'/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>restart</on_crash>
  <devices>
    <emulator>/usr/lib64/xen/bin/qemu-dm</emulator>
    <disk type='file' device='disk'>
      <driver name='file'/>
      <source file='/var/lib/xen/images/dom_test'/>
      <target dev='hda' bus='ide'/>
    </disk>
    <disk type='file' device='cdrom'>
      <driver name='file'/>
      <source
file='/tmp/en_win_srv_2003_r2_enterprise_x64_with_sp2_cd1_X13-06188.iso'/>
      <target dev='hdc' bus='ide'/>
      <readonly/>
    </disk>
    <disk type='file' device='floppy'>
      <driver name='file'/>
      <source file='/tmp/floppy.img'/>
      <target dev='fda' bus='fdc'/>
    </disk>
    <interface type='bridge'>
      <mac address='54:52:00:29:03:d7'/>
      <source bridge='virbr0'/>
      <script path='vif-bridge'/>
      <target dev='vif20.0'/>
    </interface>
    <serial type='pty'>
      <source path='/dev/pts/1'/>
      <target port='0'/>
    </serial>
    <console type='pty' tty='/dev/pts/1'>
      <source path='/dev/pts/1'/>
      <target port='0'/>
    </console>
    <input type='mouse' bus='ps2'/>
    <graphics type='vnc' port='5920' autoport='yes' keymap='en-us'/>
  </devices>
</domain>


# virsh define tmp 
Domain dom_test11 defined from tmp

3. dumpxml for defined guest
# virsh dumpxml dom_test11
<domain type='xen'>
  <name>dom_test11</name>
  <uuid>d8c03f55-b1ca-eaa6-4fef-6f731af3b01f</uuid>
  <memory>1048576</memory>
  <currentMemory>1048576</currentMemory>
  <vcpu>1</vcpu>
  <os>
    <type arch='x86_64' machine='xenfv'>hvm</type>
    <loader>/usr/lib/xen/boot/hvmloader</loader>
    <boot dev='cdrom'/>
  </os>
  <features>
    <acpi/>
    <apic/>
    <pae/>
  </features>
  <clock offset='utc'/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>restart</on_crash>
  <devices>
    <emulator>/usr/lib64/xen/bin/qemu-dm</emulator>
    <disk type='file' device='disk'>
      <driver name='file'/>
      <source file='/var/lib/xen/images/dom_test'/>
      <target dev='hda' bus='ide'/>
    </disk>
    <disk type='file' device='cdrom'>
      <driver name='file'/>
      <source
file='/tmp/en_win_srv_2003_r2_enterprise_x64_with_sp2_cd1_X13-06188.iso'/>
      <target dev='hdc' bus='ide'/>
      <readonly/>
    </disk>
    <interface type='bridge'>
      <mac address='54:52:00:29:03:d7'/>
      <source bridge='virbr0'/>
      <script path='vif-bridge'/>
      <target dev='vif20.0'/>
    </interface>
    <serial type='pty'>
      <target port='0'/>
    </serial>
    <console type='pty'>
      <target port='0'/>
    </console>
    <input type='mouse' bus='ps2'/>
    <graphics type='vnc' port='-1' autoport='yes' keymap='en-us'/>
  </devices>
</domain>

This request was evaluated by Red Hat Product Management for inclusion in Red Hat Enterprise Linux 5.7 and Red Hat does not plan to fix this issue the currently developed update.

Contact your manager or support representative in case you need to escalate this bug.

Development Management has reviewed and declined this request.  You may appeal
this decision by reopening this request.