Add c_pointer method to classes.

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