target/riscv/kvm: do PR_RISCV_V_SET_CONTROL during realize()

target/riscv/kvm: do PR_RISCV_V_SET_CONTROL during realize()

Linux RISC-V vector documentation (Document/arch/riscv/vector.rst)
mandates a prctl() in order to allow an userspace thread to use the
Vector extension from the host.

This is something to be done in realize() time, after init(), when we
already decided whether we're using RVV or not. We don't have a
realize() callback for KVM yet, so add kvm_cpu_realize() and enable RVV
for the thread via PR_RISCV_V_SET_CONTROL.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20231218204321.75757-4-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>