qapi: merge QInt and QFloat in QNum

qapi: merge QInt and QFloat in QNum

We would like to use a same QObject type to represent numbers, whether
they are int, uint, or floats. Getters will allow some compatibility
between the various types if the number fits other representations.

Add a few more tests while at it.

Signed-off-by: Marc-André Lureau <[email protected]>
Message-Id: <20170607163635[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
[parse_stats_intervals() simplified a bit, comment in
test_visitor_in_int_overflow() tidied up, suppress bogus warnings]
Signed-off-by: Markus Armbruster <[email protected]>