There a number of cases where Coverity has spotted writes to variables
when the written values wouldn't be used subsequently. The patches here
are independent of one another, except for this common theme.

1: xen-mfndump: drop dead assignment to "page" from lookup_pte_func()
2: libxl: drop dead assignments to "ret" from libxl__domain_config_setdefault()
3: libxg: drop dead assignment to "ptes" from xc_core_arch_map_p2m_list_rw()
4: libxg: drop dead assignment to "rc" from xc_cpuid_apply_policy()
5: libxl: drop dead assignment to transaction variable from libxl__domain_make()

Jan