KVM: arm64: PMU: Don't advertise STALL_SLOT_{FRONTEND,BACKEND}

Don't advertise STALL_SLOT_{FRONT,BACK}END events to the guest,
similar to STALL_SLOT event, as when any of these three events
are implemented, all three of them should be implemented,
according to the Arm ARM.

Suggested-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Reiji Watanabe <reijiw@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230819043947.4100985-5-reijiw@google.com