tools/xenalyze: Fix build with clang

tools/xenalyze: Fix build with clang

1) EXIT_REASON_EXCEPTION_NMI is 0, and Clang complains:

xenalyze.c:513:33: error: initializer overrides prior initialization of this subobject [-Werror,-Winitializer-overrides]
    [EXIT_REASON_EXCEPTION_NMI]="EXCEPTION_NMI",
                                ^~~~~~~~~~~~~~~
xenalyze.c:512:11: note: previous initialization is here
    [0] = "NONE",
          ^~~~~~

2) cr3_time_compare(), eip_compare(), ipi_send() and cr3_compare_start() are
   declared as nested functions, which is a GCCism not supported by Clang.

   As they don't actually make use of the interesting feature offered by
   nested functions (i.e. dynamic scoping), move them to just being normal
   functions.

3) clear_interval_summary(), update_cpi() and clear_interval_cpi() are all
   unused.  The former isn't reference anywhere, so is deleted, while the other
   two are called from currently #if 0'd code.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>