RISC-V: KVM: Define a probe function for SBI extension data structures

Currently the probe function just checks if an SBI extension is
registered or not. However, the extension may not want to advertise
itself depending on some other condition.
An additional extension specific probe function will allow
extensions to decide if they want to be advertised to the caller or
not. Any extension that does not require additional dependency checks
can avoid implementing this function.

Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Signed-off-by: Anup Patel <anup@brainfault.org>