arm64: kvm: trace_handle_exit: use string choices helper

We can use string choices helper, let's use it.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/87o6ti5ksx.wl-kuninori.morimoto.gx@renesas.com


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