virt-host-validate: Fix warning for IOMMU detection on PPC

virt-host-validate: Fix warning for IOMMU detection on PPC

Fix the warning generated on PPC by virt-host-validate for IOMMU.
In case of PPC, IOMMU in the host kernel either has it or it's not
compiled in. The /sys/kernel/iommu_groups check is good enough to
verify if it was compiled with the kernel or not.

Modify the error message when "if (sb.st_nlink <= 2)" to indicate
what the problem would be since there would be no @bootarg.

Signed-off-by: Nitesh Konkar <nitkon12@linux.vnet.ibm.com>