ANDROID: KVM: arm64: pkvm: Refactor DABT entry/exit

Instruction and data aborts taken from a guest are very different
beasts, especially as MMIO exits are only ever triggered by the latter.

Rework the shadow handlers so that instruction and data aborts are
handled independently, making the control flow and MMIO handling
considerably easier to reason about.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Bug: 209580772
Change-Id: I8a0d6881a13154ceaedf4b51ff224256db2e7ebf
Signed-off-by: Will Deacon <willdeacon@google.com>