Handle unreadable /proc/xen/capabilities

Fixes #1854.

setup

@@ -713,10 +713,12 @@ class LinuxVirtual(Virtual):
         if os.path.exists("/proc/xen"):
             self.facts['virtualization_type'] = 'xen'
             self.facts['virtualization_role'] = 'guest'
-            if os.path.exists('/proc/xen/capabilities'):
+            try:
                 for line in open('/proc/xen/capabilities'):
                     if "control_d" in line:
                         self.facts['virtualization_role'] = 'host'
+            except IOError:
+                pass
             return
  
         if os.path.exists('/proc/vz'):