target/arm/kvm: Set PMU for host only when available

target/arm/kvm.c checked PMU availability but unconditionally set the
PMU feature flag for the host CPU model, which is confusing. Set the
feature flag only when available.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>