target/riscv/kvm: improve 'init_multiext_cfg' error msg

target/riscv/kvm: improve 'init_multiext_cfg' error msg

Our error message is returning the value of 'ret', which will be always
-1 in case of error, and will not be that useful:

qemu-system-riscv64: Unable to read ISA_EXT KVM register ssaia, error -1

Improve the error message by outputting 'errno' instead of 'ret'. Use
strerrorname_np() to output the error name instead of the error code.
This will give us what we need to know right away:

qemu-system-riscv64: Unable to read ISA_EXT KVM register ssaia, error code: ENOENT

Given that we're going to exit(1) in this condition instead of
attempting to recover, remove the 'kvm_riscv_destroy_scratch_vcpu()'
call.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20231003132148.797921-2-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>