conf: Format and parse backing chains in domain XML

conf: Format and parse backing chains in domain XML

This patch implements formating and parsing code for the backing store
schema defined and documented by the previous patch.

This patch does not aim at providing full persistent storage of disk
backing chains yet. The formatter is supposed to provide the backing
chain detected when starting a domain and thus it is not formatted into
an inactive domain XML. The parser is implemented mainly for the purpose
of testing the XML generated by the formatter and thus it does not
distinguish between no backingStore element and an empty backingStore
element. This will have to change once we fully implement support for
user-supplied backing chains.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>