target/riscv/kvm: get/set vector vregs[]

target/riscv/kvm: get/set vector vregs[]

vregs[] have variable size that depends on the current vlenb set by the
host, meaning we can't use our regular kvm_riscv_reg_id() to retrieve
it.

Create a generic kvm_encode_reg_size_id() helper to encode any given
size in bytes into a given kvm reg id. kvm_riscv_vector_reg_id() will
use it to encode vlenb into a given vreg ID.

kvm_riscv_(get|set)_vector() can then get/set all 32 vregs.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20240123161714.160149-4-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>