util: hostcpu: improve CPU freq code for FreeBSD

Current implementation uses the dev.cpu.0.freq sysctl that is
provided by the cpufreq(4) framework and returns the actual
CPU frequency. However, there are environments where it's not available,
e.g. when running nested in KVM. In this case fall back to hw.clockrate
that reports CPU frequency at the boot time.

Resolves (hopefully):
https://bugzilla.redhat.com/show_bug.cgi?id=1369964

src/util/virhostcpu.c

@@ -994,9 +994,16 @@ virHostCPUGetInfo(virArch hostarch ATTRIBUTE_UNUSED,
     *threads = 1;
 
 # ifdef __FreeBSD__
+    /* dev.cpu.%d.freq reports current active CPU frequency. It is provided by
+     * the cpufreq(4) framework. However, it might be disabled or no driver
+     * available. In this case fallback to "hw.clockrate" which reports boot time
+     * CPU frequency. */
+
     if (sysctlbyname("dev.cpu.0.freq", &cpu_freq, &cpu_freq_len, NULL, 0) < 0) {
-        virReportSystemError(errno, "%s", _("cannot obtain CPU freq"));
-        return -1;
+        if (sysctlbyname("hw.clockrate", &cpu_freq, &cpu_freq_len, NULL, 0) < 0) {
+            virReportSystemError(errno, "%s", _("cannot obtain CPU freq"));
+            return -1;
+        }
     }
 
     *mhz = cpu_freq;