KVM: arm64: nv: Respect exception routing rules for SEAs

Synchronous external aborts are taken to EL2 if ELIsInHost() or
HCR_EL2.TEA=1. Rework the SEA injection plumbing to respect the imposed
routing of the guest hypervisor and opportunistically rephrase things to
make their function a bit more obvious.

Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20250708172532.1699409-6-oliver.upton@linux.dev


Signed-off-by: Oliver Upton <oliver.upton@linux.dev>