arm64: KVM: Add Virtual Abort injection helper

Now that we're able to context switch the HCR_EL2.VA bit, let's
introduce a helper that injects an Abort into a vcpu.

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