Description of problem:
Console not configured for guest after adding second graphics device.

Version-Release number of selected component (if applicable):
virt-manager-0.6.1-15.el5
libvirt-0.8.2.24.el5
python-virtinst-0.400.3-13.el5
xen-3.0.3-135.el5

# uname -a
Linux wjw.redhat.com 2.6.18-294.el5xen #1 SMP Tue Oct 25 16:39:47 EDT 2011 x86_64 x86_64 x86_64 GNU/Linux

How reproducible:
always

Steps to Reproduce:
1.Launch virt-manager:
#virt-manager --debug --no-fork
2.Make sure a good guest is in shutdown status and with vnc graphics:
#virsh dumpxml 61rhel
.......
</console>
    <input type='mouse' bus='xen'/>
    <graphics type='vnc' port='5900' autoport='yes' listen='127.0.0.1' keymap='en-us'/>
.........

3. Click the hardware tab in the Virtual Machine Details window to add second vnc graphics,there is no message pop up.
   Add Hardware - > Graphics, Type as "VNC server", click Forward , and click Finish

4. Check the xml of guest 61rhel, there is no graphics info:
#virsh dumpxml 61rhel
<domain type='xen'>
  <name>61rhel</name>
  <uuid>598bdbe7-2989-1882-9f3e-a42ce93ab335</uuid>
  <memory>2097152</memory>
  <currentMemory>2097152</currentMemory>
  <vcpu>7</vcpu>
  <bootloader>/usr/bin/pygrub</bootloader>
  <os>
    <type arch='x86_64' machine='xenpv'>linux</type>
    <kernel>/var/lib/xen/boot_kernel.HGEy2B</kernel>
    <initrd>/var/lib/xen/boot_ramdisk.oktUcN</initrd>
    <cmdline>ro root=/dev/mapper/VolGroup-lv_root rd_LVM_LV=VolGroup/lv_root rd_LVM_LV=VolGroup/lv_swap rd_NO_LUKS rd_NO_MD rd_NO_DM LANG=en_US.UTF-8 SYSFONT=latarcyrheb-sun16 KEYBOARDTYPE=pc KEYTABLE=us crashkernel=auto rhgb quiet</cmdline>
  </os>
  <clock offset='utc'/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>restart</on_crash>
  <devices>
    <disk type='file' device='disk'>
      <driver name='tap' type='aio'/>
      <source file='/var/lib/xen/images/61rhel.img'/>
      <target dev='xvda' bus='xen'/>
    </disk>
    <interface type='bridge'>
      <mac address='00:16:3e:7a:7b:a7'/>
      <source bridge='virbr0'/>
      <script path='vif-bridge'/>
      <target dev='vif2.0'/>
    </interface>
    <console type='pty'>
      <target port='0'/>
    </console>
  </devices>
</domain>


5. Start the guest, display "console not configured for guest".

  
Actual results:
2011-11-15 18:52:57,511 DEBUG    Starting vm '61rhel'.
2011-11-15 18:52:59,049 DEBUG    Trying console login
2011-11-15 18:52:59,068 DEBUG    No graphics configured in guest
2011-11-15 18:52:59,092 DEBUG    VM 61rhel started
2011-11-15 18:52:59,127 ERROR    Error reading block stats internal error read_bd_stats: Failed to read any block statistics

Expected results:
virt-manager should forbid customer to add second graphics device or pop up error message to remind customer.

Additional info:
1. If create guest with VNC graphics, then add local SDL window, hit the same issue.
2. On version 6.2， Error message will pop up to remind customer only one graphics device is supported：
"Error starting domain: internal error only 1 graphics device is supported"