Ignore error when 'xen.lowlevel.xs' doesn't exists

There are still few uses of direct xenstore access, most of them are
xen-specific (so doesn't need to be portable). For now simply don't
connect to xenstore when no 'xen.lowlevel.xs' module present. It will
break such xen-specific accesses - it must be somehow reworked - either
by adding appropriate conditionals, or moving such code somewhere else
(custom methods of libvirt driver?).

core/qubes.py

@@ -42,7 +42,10 @@ dry_run = False
 
 if not dry_run:
     import libvirt
+    try:
         import xen.lowlevel.xs
+    except ImportError:
+        pass
 
 
 qubes_base_dir   = "/var/lib/qubes"
@@ -138,6 +141,7 @@ class QubesVMMConnection(object):
             # Do not initialize in offline mode
             return
 
+        if 'xen.lowlevel.xs' in sys.modules:
             self._xs = xen.lowlevel.xs.xs()
         self._libvirt_conn = libvirt.open(defaults['libvirt_uri'])
         if self._libvirt_conn == None:
@@ -160,7 +164,10 @@ class QubesVMMConnection(object):
 
     @property
     def xs(self):
+        if 'xen.lowlevel.xs' in sys.modules:
             return self._common_getter('_xs')
+        else:
+            return None
 
 
 ##### VMM global variable definition #####