[Backport] KVM: add 'vlenb' Vector CSR

[yechao-w]

配置CONFIG_RISCV_ISA_V=y后，启动kvm虚拟机报错：

qemu-system-riscv64: Unable to read vlenb register, error code: 2

kvm one reg 增加对vlenb的支持。upstream补丁：

197bd23 RISC-V: KVM: set 'vlenb' in kvm_riscv_vcpu_alloc_vector_context()
2fa2903 RISC-V: KVM: add 'vlenb' Vector CSR
3975525 RISC-V: KVM: add vector registers and CSRs in KVM_GET_REG_LIST (已在 76c3bf6 合入)
7997662 RISC-V: KVM: fix stack overrun when loading vlenb