RISC-V: KVM: Introduce optional ONE_REG callbacks for SBI extensions

RISC-V: KVM: Introduce optional ONE_REG callbacks for SBI extensions

SBI extensions can have per-VCPU state which needs to be saved/restored
through ONE_REG interface for Guest/VM migration. Introduce optional
ONE_REG callbacks for SBI extensions so that ONE_REG implementation
for an SBI extenion is part of the extension sources.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20250823155947.1354229-4-apatel@ventanamicro.com
Signed-off-by: Anup Patel <anup@brainfault.org>