KVM: arm64: nv: Expose FEAT_EVT to nested guests

Now that we properly implement FEAT_EVT (as we correctly forward
traps), expose it to guests.

Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Jing Zhang <jingzhangos@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230815183903.2735724-17-maz@kernel.org