target/riscv/kvm: do not read unavailable CSRs

target/riscv/kvm: do not read unavailable CSRs

[1] reports that commit 4db19d5b21 broke a KVM guest running kernel 6.6.
This happens because the kernel does not know 'senvcfg', making it
unable to boot because QEMU is reading/wriiting it without any checks.

After converting the CSRs to do "automated" get/put reg procedures in
the previous patch we can now scan for availability. Two functions are
created:

- kvm_riscv_read_csr_cfg_legacy() will check if the CSR exists by brute
  forcing KVM_GET_ONE_REG in each one of them, interpreting an EINVAL
  return as indication that the CSR isn't available. This will be use in
  absence of KVM_GET_REG_LIST;

- kvm_riscv_read_csr_cfg() will use the existing result of get_reg_list
  to check if the CSRs ids are present.

kvm_riscv_init_multiext_cfg() is now kvm_riscv_init_cfg() to reflect that
the function is also dealing with CSRs.

[1] https://lore.kernel.org/qemu-riscv/CABJz62OfUDHYkQ0T3rGHStQprf1c7_E0qBLbLKhfv=+jb0SYAw@mail.gmail.com/

Fixes: 4db19d5b21 ("target/riscv/kvm: add missing KVM CSRs")
Reported-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250429124421.223883-7-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Cc: qemu-stable@nongnu.org