nss: convert findLeases to use json-c

nss: convert findLeases to use json-c

While the parsing is still done by 1K buffers, the results
are no longer filtered during the parsing, but the whole JSON
has to live in memory at once, which was also the case before
the NSS plugin dropped its dependency on libvirt_util.

Also, the new parser might be more forgiving of missing elements.

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>