arm: KVM: Allow an exit code to be tagged with a Virtual Abort

An asynchronous abort can also be triggered whilst running at EL2.
But instead of making that a new error code, we need to communicate
it to the rest of KVM together with the exit reason.
So let's hijack a single bit that allows the exception code to be
tagged with a "pending Abort" information.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>