This patch adds API to modify domain metadata for running and stopped
domains. The api supports changing description, title as well as the
newly added <metadata> element. The API has support for storing data in
the metadata element using xml namespaces.

* include/libvirt/libvirt.h.in
* src/libvirt_public.syms
        - add function headers
        - add enum to select metadata to operate on
        - export functions
* src/libvirt.c
        - add public api implementation
* src/driver.h
        - add driver support
* src/remote/remote_driver.c
* src/remote/remote_protocol.x
        - wire up the remote protocol
* include/libvirt/virterror.h
* src/util/virterror.c
        - add a new error message note that metadata for domain are
        missing