target/riscv: th: make CSR insertion test a bit more intuitive

target/riscv: th: make CSR insertion test a bit more intuitive

In preparation for generalizing the custom CSR functionality,
make the test return bool instead of int.  Make the insertion_test
optional, too.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>