qapi: Replace dirty is_c_ptr() by method c_null()

is_c_ptr() looks whether the end of the C text for the type looks like
a pointer.  Works, but is fragile.

We now have a better tool: use QAPISchemaType method c_null().  The
initializers for non-pointers become prettier: 0, false or the
enumeration constant with the value 0 instead of {0}.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <1442401589-24189-13-git-send-email-armbru@redhat.com>
