KVM: arm64: PMU: Don't advertise the STALL_SLOT event

Currently, KVM hides the STALL_SLOT event for guests if the
host PMU version is PMUv3p4 or newer, as PMMIR_EL1 is handled
as RAZ for the guests. But, this should be based on the guests'
PMU version (instead of the host PMU version), as an older PMU
that doesn't support PMMIR_EL1 could support the STALL_SLOT
event, according to the Arm ARM. Exposing the STALL_SLOT event
without PMMIR_EL1 won't be very useful anyway though.

Stop advertising the STALL_SLOT event for guests unconditionally,
rather than fixing or keeping the inaccurate checking to
advertise the event for the case, where it is not very useful.

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/20230819043947.4100985-4-reijiw@google.com