RISC-V: KVM: Implement get event info function

RISC-V: KVM: Implement get event info function

The new get_event_info funciton allows the guest to query the presence
of multiple events with single SBI call. Currently, the perf driver
in linux guest invokes it for all the standard SBI PMU events. Support
the SBI function implementation in KVM as well.

Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Acked-by: Paul Walmsley <pjw@kernel.org>
Link: https://lore.kernel.org/r/20250909-pmu_event_info-v6-7-d8f80cacb884@rivosinc.com
Signed-off-by: Anup Patel <anup@brainfault.org>