RISC-V: KVM: Convert kvm_riscv_vcpu_sbi_forward() into extension handler

RISC-V: KVM: Convert kvm_riscv_vcpu_sbi_forward() into extension handler

All uses of kvm_riscv_vcpu_sbi_forward() also updates retdata->uexit so
to further reduce code duplication move retdata->uexit assignment to
kvm_riscv_vcpu_sbi_forward() and convert it into SBI extension handler.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20251017155925.361560-2-apatel@ventanamicro.com
Signed-off-by: Anup Patel <anup@brainfault.org>