qapi: Allow true, false and null in schema json

In the near term, we will use it for a sensible-looking
'gen':false inside command declarations, instead of the
current ugly 'gen':'no'.

In the long term, it will allow conversion from shorthand
with defaults mentioned only in side-band documentation:
 'data':{'*flag':'bool', '*string':'str'}
into an explicit default value documentation, as in:
 'data':{'flag':{'type':'bool', 'optional':true, 'default':true},
         'string':{'type':'str', 'optional':true, 'default':null}}

We still don't parse integer values (also necessary before
we can allow explicit defaults), but that can come in a later
series.

Update the testsuite to match an improved error message.

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

tests/qapi-schema/bad-type-bool.err

@@ -1 +1 @@
-tests/qapi-schema/bad-type-bool.json:3:11: Stray "t"
+tests/qapi-schema/bad-type-bool.json:2: 'type' key must have a string value

tests/qapi-schema/bad-type-bool.json

@@ -1,3 +1,2 @@
 # we reject an expression with a metatype that is not a string
-# FIXME: once the parser understands bool inputs, improve the error message
 { 'type': true, 'data': { } }