Add c_pointer method to classes.

This returns the raw C pointer to the underlying object, eg:

  conn = libvirt.open(None)
  print "0x%x" % conn.c_pointer()   # returns virConnectPtr of the connection
  dom = conn.lookupByName("test")
  print "0x%x" % dom.c_pointer()    # returns virDomainPtr of the domain

The reason behind this is to allow us to transparently pass Python dom
objects through the libguestfs Python API.

https://bugzilla.redhat.com/show_bug.cgi?id=1075164

sanitytest.py

@@ -109,6 +109,8 @@ for klassname in gottypes:
     for name in dir(klassobj):
         if name[0] == '_':
             continue
+        if name == 'c_pointer':
+            continue
         thing = getattr(klassobj, name)
         if callable(thing):
             gotfunctions[klassname].append(name)