Description of problem:
When trying to build an EC2 upload style RHEL-5 i386 image, the image factory throws an exception:


2011-08-11 10:08:18,795 INFO imgfac.builders.BaseBuilder.RHEL5_ec2_Builder pid(1858) Message: Modifying and updating menu.lst
2011-08-11 10:08:18,808 DEBUG imgfac.builders.BaseBuilder.RHEL5_ec2_Builder pid(1858) Message: Exception caught in ImageFactory
2011-08-11 10:08:18,827 DEBUG imgfac.builders.BaseBuilder.RHEL5_ec2_Builder pid(1858) Message: Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/imgfac/builders/Fedora_ec2_Builder.py", line 170, in build_upload
    self.ec2_modify_filesystem()
  File "/usr/lib/python2.7/site-packages/imgfac/builders/Fedora_ec2_Builder.py", line 387, in ec2_modify_filesystem
    raise ImageFactoryException("Unable to extract kernel version")
ImageFactoryException: Unable to extract kernel version


After talking to Ian on IRC, the problem is that the factory goes looking for an installed Xen kernel during ec2_modify_filesystem().  If you didn't explicitly specify that in the TDL, you will hit this message.  My thinking is that we should probably force install the -xen kernel when we know we are doing EC2 upload style RHEL-5 i386 builds.