RISC-V: KVM: add SBI extension init()/deinit() functions

RISC-V: KVM: add SBI extension init()/deinit() functions

The FWFT SBI extension will need to dynamically allocate memory and do
init time specific initialization. Add an init/deinit callbacks that
allows to do so.

Signed-off-by: Clément Léger <cleger@rivosinc.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Link: https://lore.kernel.org/r/20250523101932.1594077-12-cleger@rivosinc.com
Signed-off-by: Anup Patel <anup@brainfault.org>