v2v: -o local: Use /var/tmp instead of /tmp for examples.

In case people are suffering from broken tmp-on-tmpfs setups.
v2v: -i libvirtxml: Add example libvirt XML.
Enable warnings in C code linked to virt-sparsify and virt-v2v.

The C code didn't have -Werror enabled (when configured) and didn't
enable the full set of warnings we would want.

However OCaml-C bindings commonly declare functions which are
called directly from OCaml, eg:

  external parse_memory : string -> doc = "v2v_xml_parse_memory"

  value v2v_xml_parse_memory (value xmlv)

These do not require prototypes as no other C code will call them, so
we have to switch off the GCC -Werror=missing-prototypes flag for
these files.

Also fixes some confusion between _CPPFLAGS and _CFLAGS.
v2v: add xmlXPathRegisterNs for libxml2 binding

Signed-off-by: Shahar Havivi <shaharh@redhat.com>