Avoid warning message from libxl driver on non-Xen kernels

The libxl driver reads /proc/xen/capabilities to see if it
is on a Dom0 kernel. If that file does not even exist though,
an error is logged. Check for the file existance before trying
to read its contents to avoid the log message.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

src/libxl/libxl_driver.c

@@ -66,6 +66,8 @@
 #define LIBXL_CONFIG_FORMAT_XM "xen-xm"
 #define LIBXL_CONFIG_FORMAT_SEXPR "xen-sxpr"
 
+#define HYPERVISOR_CAPABILITIES "/proc/xen/capabilities"
+
 /* Number of Xen scheduler parameters */
 #define XEN_SCHED_CREDIT_NPARAM   2
 
@@ -970,12 +972,17 @@ libxlDriverShouldLoad(bool privileged)
         return ret;
     }
 
+    if (!virFileExists(HYPERVISOR_CAPABILITIES)) {
+        VIR_INFO("Disabling driver as " HYPERVISOR_CAPABILITIES
+                 " does not exist");
+        return false;
+    }
     /*
      * Don't load if not running on a Xen control domain (dom0). It is not
      * sufficient to check for the file to exist as any guest can mount
      * xenfs to /proc/xen.
      */
-    status = virFileReadAll("/proc/xen/capabilities", 10, &output);
+    status = virFileReadAll(HYPERVISOR_CAPABILITIES, 10, &output);
     if (status >= 0) {
         status = strncmp(output, "control_d", 9);
     }