tools/libxl: Code-gen improvements for libxl_save_msgs_gen.pl

tools/libxl: Code-gen improvements for libxl_save_msgs_gen.pl

our @msgs() is an array of $msginfo's where the first element is a
unique number.  The $msgnum_used check ensures they are unique.  Instead
if specifying them explicitly, generate msgnum locally.  This reduces
the diff necessary to edit the middle of the @msgs() array.

All other hunks are adjusting formatting in the generated C, to make it
easier to follow.

No change in behaviour of the generated C.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>