arm: KVM: Preserve pending Virtual Abort in world switch

The HCR.VA bit is used to signal an Abort to a guest, and has
the peculiar feature of getting cleared when the guest has taken
the abort (this is the only bit that behaves as such in this register).

This means that if we signal such an abort, we must leave it in
the guest context until it disappears from HCR, and at which point
it must be cleared from the context.

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