Description of problem:
using cache=writeback or cache=writethrough in --disk option will get an error on xenfv of virt-install

Version-Release number of selected component (if applicable):
python-virtinst-0.400.3-12.el5

How reproducible:
always

Steps to Reproduce:
1. run command line:
#virt-install -n demo -r 1024 --disk path=/tmp/demo.img,size=5,device=disk,cache=writeback --hvm -c /dev/cdrom
 or run command line:
#virt-install -n demo -r 1024 --disk path=/tmp/demo.img,size=5,device=disk,cache=writethrough --hvm -c /dev/cdrom
  
Actual results:
ERROR    POST operation failed: xend_post: error from xen daemon: (xend.err 'Error creating domain: Block device type "qemu" is invalid.')
Domain installation may not have been
 successful.  If it was, you can restart your domain
 by running 'virsh start demo'; otherwise, please
 restart your installation.
ERROR    POST operation failed: xend_post: error from xen daemon: (xend.err 'Error creating domain: Block device type "qemu" is invalid.')
Traceback (most recent call last):
  File "/usr/sbin/virt-install", line 895, in ?
    main()
  File "/usr/sbin/virt-install", line 757, in main
    start_time, guest.start_install)
  File "/usr/sbin/virt-install", line 819, in do_install
    dom = install_func(conscb, progresscb, wait=(not wait))
  File "/usr/lib/python2.4/site-packages/virtinst/Guest.py", line 541, in start_install
    return self._do_install(consolecb, meter, removeOld, wait)
  File "/usr/lib/python2.4/site-packages/virtinst/Guest.py", line 633, in _do_install
    self.domain = self.conn.createLinux(install_xml, 0)
  File "/usr/lib64/python2.4/site-packages/libvirt.py", line 1277, in createLinux
    if ret is None:raise libvirtError('virDomainCreateLinux() failed', conn=self)
libvirtError: POST operation failed: xend_post: error from xen daemon: (xend.err 'Error creating domain: Block device type "qemu" is invalid.')


Expected results:
install will be successful

Additional info: