older rhel/centos/FC guests on fc6 host and non working console solved

I spend some time figuring out why my old centos/rhel/fc images I converted
for xen didnt have a proper serial console. It turns out they have either
a missing or older version of /dev/xvc0.

The fix is to make sure the new device eixsts. either in the initrd and/or
some other place. Since these older versions of the OS are likely booting
with a newer kernel (from your FC6 host), you have a non working udev, so
you need to make sure the device exists manually:

mknod /dev/xvc0 c 250 187

Be aware that there are many wrong/old google hits on the majore/minors
of this.

I hope this helps others not waste as much time as me :)

Paul