--enable-kvm not work for riscv64-softmmu
Description of problem:
I want to enable kvm for qemu-system-riscv64, so I compile it with `--enable-kvm` as above. But the log shows

```sh
  Targets and accelerators
    KVM support                  : NO
```

And also compiled qemu-system-riscv64 does not support kvm.
Steps to reproduce:
1. clone the repo
2. `./configure --target-list=riscv64-softmmu --enable-kvm`