target/riscv: move 'vlen' to riscv_cpu_properties[]

target/riscv: move 'vlen' to riscv_cpu_properties[]

Turning 'vlen' into a class property will allow its default value to be
overwritten by cpu_init() later on, solving the issue we have now where
CPU specific settings are getting overwritten by the default.

Common validation bits are moved from riscv_cpu_validate_v() to
prop_vlen_set() to be shared with KVM.

And, as done with every option we migrated to riscv_cpu_properties[],
vendor CPUs can't have their 'vlen' value changed.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Tested-by: Vladimir Isaev <vladimir.isaev@syntacore.com>
Message-ID: <20240105230546.265053-9-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>