xen/common: address MISRA C:2012 Rule 5.3

xen/common: address MISRA C:2012 Rule 5.3

The following strategies are adopted to deal with violations
of MISRA C:2012 Rule 5.3:
"An identifier declared in an inner scope shall not hide an
identifier declared in an outer scope".

- s/nodes/numa_nodes/ for the file-scope variable in 'common/numa.c';
- move the variable 'struct compat_remove_from_physmap cmp' inside
  the outer union variable 'cmp' to avoid shadowing it.

Signed-off-by: Nicola Vetrini <nicola.vetrini@bugseng.com>
Acked-by: Jan Beulich <jbeulich@suse.com>