tools/libxc: Introduce ARRAY_SIZE() and replace handrolled examples

tools/libxc: Introduce ARRAY_SIZE() and replace handrolled examples

xen-hptool and xen-mfndump include xc_private.h.  This is bad, but not trivial
to fix, so they gain a protective #undef and a stern comment.

MiniOS leaks ARRAY_SIZE into the libxc namespace as part of a stubdom build.
Therefore, xc_private.h gains an #ifndef until MiniOS is fixed.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>