dom0/core: get timezone from /etc/localtime symlink

Fedora 18 doesn't have /etc/sysconfig/clock. Instead have /etc/localtime
symlinked to real timezone (instead of hardlinked like before), so now it is
easy to get destination TZ name.

dom0/qvm-core/qubes.py

@@ -831,6 +831,11 @@ class QubesVm(object):
 
     # FIXME: should be outside of QubesVM?
     def get_timezone(self):
+        # fc18
+        if os.path.islink('/etc/localtime'):
+            return '/'.join(os.readlink('/etc/localtime').split('/')[-2:])
+        # <=fc17
+        elif os.path.exists('/etc/sysconfig/clock'):
             clock_config = open('/etc/sysconfig/clock', "r")
             clock_config_lines = clock_config.readlines()
             clock_config.close()