RISC-V: KVM: Improve vector save/restore errors

kvm_riscv_vcpu_(get/set)_reg_vector() now returns ENOENT if V is not
available, EINVAL if reg type is not of VECTOR type, and any error that
might be thrown by kvm_riscv_vcpu_vreg_addr().

Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Anup Patel <anup@brainfault.org>