KVM: arm64: PMU: Narrow the overflow checking when required

For 64bit counters that overflow on a 32bit boundary, make
sure we only check the bottom 32bit to generate a CHAIN event.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Reiji Watanabe <reijiw@google.com>
Link: https://lore.kernel.org/r/20221113163832.3154370-6-maz@kernel.org