latest xen update breaks boot for non iommu

[Mike Wright]

Hi all,

The latest f18 yum update has a broken boot stanza in grub2 when booting 
xen.

The assumption is that hardware iommu exists.  If your system doesn't 
have hardware iommu your boot WILL fail.

If you encounter this situation change the following line in grub.cfg

module  /xen.gz /xen.gz placeholder   iommu=force

to:

module  /xen.gz /xen.gz placeholder   iommu=soft