AIL=2 mode has no known users, so is not well tested or supported.
Disallow guests from selecting this mode because it may become
deprecated in future versions of the architecture.

This policy decision is not left to QEMU because KVM support is
required for AIL=2 (when injecting interrupts).
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>