RISC-V: KVM: set 'vlenb' in kvm_riscv_vcpu_alloc_vector_context()

'vlenb', added to riscv_v_ext_state by commit c35f3aa3

 ("RISC-V:
vector: export VLENB csr in __sc_riscv_v_state"), isn't being
initialized in guest_context. If we export 'vlenb' as a KVM CSR,
something we want to do in the next patch, it'll always return 0.

Set 'vlenb' to riscv_v_size/32.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Anup Patel <anup@brainfault.org>