lib: Revert removing of Summary and Description fields in headers

Those fields are used by the apibuild script to create documentation.
This commit partialy reverts a4bcefbc.

include/libvirt/libvirt-qemu.h

 /* -*- c -*-
  * libvirt-qemu.h: Interfaces specific for QEMU/KVM driver
+ * Summary: qemu specific interfaces
+ * Description: Provides the interfaces of the libvirt library to handle
+ *              qemu specific methods
  *
  * Copyright (C) 2010, 2012 Red Hat, Inc.
  *

include/libvirt/libvirt.h.in

 /* -*- c -*-
  * libvirt.h: Core interfaces for the libvirt library
+ * Summary: core interfaces for the libvirt library
+ * Description: Provides the interfaces of the libvirt library to handle
+ *              virtualized domains
  *
  * Copyright (C) 2005-2006, 2010-2012 Red Hat, Inc.
  *

include/libvirt/virterror.h

 /*
  * virterror.h: Error handling interfaces for the libvirt library
+ * Summary: error handling interfaces for the libvirt library
+ * Description: Provides the interfaces of the libvirt library to handle
+ *              errors raised while using the library.
  *
  * Copyright (C) 2006, 2010-2012 Red Hat, Inc.
  *