builder: Fix RHEL script for RHEL 5.

Root partition is on /dev/sda2:

$ virt-filesystems -a rhel-5.10 --all --long -h
Name       Type        VFS   Label      MBR  Size  Parent
/dev/sda1  filesystem  ext2  /boot      -    510M  -
/dev/sda2  filesystem  ext4  /          -    4.5G  -
/dev/sda3  filesystem  swap  SWAP-hda3  -    1.0G  -
/dev/sda1  partition   -     -          83   510M  /dev/sda
/dev/sda2  partition   -     -          83   4.5G  /dev/sda
/dev/sda3  partition   -     -          82   1.0G  /dev/sda
/dev/sda   device      -     -          -    6.0G  -

It would be better if we could mount by label in guestfish ...

There is another problem with RHEL 5 guests in that the
yum URL is slightly wrong.