tools/libxl: Alter the use of rand() in testidl

tools/libxl: Alter the use of rand() in testidl

Coverity warns for every occurrence of rand(), which is made worse
because each time the IDL changes, some of the calls get re-flagged.

Collect all calls to rand() in a single function, test_rand(), which
takes a modulo parameter for convenience.  This turns 40 defects
currently into 1, which won't get re-flagged when the IDL changes.

In addition, fix the erroneous random choice for libxl_defbool_set().
"!!rand() % 1" is unconditionally 0, and even without the "% 1" would
still be very heavily skewed in one direction.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>