storage: improve handling volume export

1. Add a helper function on vm.storage. This is equivalent of:

    vm.storage.get_pool(vm.volumes[name]).export(vm.volumes[name])

2. Make sure the path returned by `export` on LVM volume is accessible.

qubes/storage/__init__.py

@@ -463,6 +463,15 @@ class Storage(object):
                     for target in parsed_xml.xpath(
                         "//domain/devices/disk/target")])
 
+    def export(self, volume):
+        ''' Helper function to export volume (pool.export(volume))'''
+        assert isinstance(volume, (Volume, basestring)), \
+            "You need to pass a Volume or pool name as str"
+        if isinstance(volume, Volume):
+            return self.pools[volume.name].export(volume)
+        else:
+            return self.pools[volume].export(self.vm.volumes[volume])
+
 
 class Pool(object):
     ''' A Pool is used to manage different kind of volumes (File

qubes/storage/lvm.py

@@ -100,7 +100,12 @@ class ThinPool(qubes.storage.Pool):
 
     def export(self, volume):
         ''' Returns an object that can be `open()`. '''
-        return '/dev/' + volume.vid
+        devpath = '/dev/' + volume.vid
+        if not os.access(devpath, os.R_OK):
+            # FIXME: convert to udev rules, and drop after introducing qubesd
+            subprocess.check_call(['sudo', 'chgrp', 'qubes', devpath])
+            subprocess.check_call(['sudo', 'chmod', 'g+rw', devpath])
+        return devpath
 
     def init_volume(self, vm, volume_config):
         ''' Initialize a :py:class:`qubes.storage.Volume` from `volume_config`.