test(run-qemu): check if /dev/kvm is readable and writable

In case `/dev/kvm` is not readable or writable, enabling KVM will fail:

```
run-qemu: timeout --foreground 600 /usr/bin/qemu-system-aarch64 -enable-kvm -cpu host [...]
qemu-system-aarch64: Could not access KVM kernel module: Permission denied
qemu-system-aarch64: failed to initialize kvm: Permission denied
```

Author: bdrung

test/run-qemu

@@ -129,7 +129,7 @@ set_vmlinux_env() {
 [[ -x /usr/bin/qemu ]] && BIN=/usr/bin/qemu && ARGS=(-cpu "$QEMU_CPU")
 (lsmod | grep -q '^kqemu ') && BIN=/usr/bin/qemu && ARGS=(-kernel-kqemu -cpu host)
 [[ -x "/usr/bin/qemu-system-${ARCH}" ]] && BIN="/usr/bin/qemu-system-${ARCH}" && ARGS=(-cpu "$QEMU_CPU")
-if [[ -z ${NO_KVM-} && -c /dev/kvm ]]; then
+if [[ -z ${NO_KVM-} && -c /dev/kvm && -r /dev/kvm && -w /dev/kvm ]]; then
     [[ -x /usr/bin/kvm ]] && BIN=/usr/bin/kvm && ARGS=(-cpu host)
     [[ -x /usr/bin/qemu-kvm ]] && BIN=/usr/bin/qemu-kvm && ARGS=(-cpu host)
     [[ -x /usr/libexec/qemu-kvm ]] && BIN=/usr/libexec/qemu-kvm && ARGS=(-cpu host)