QAPISchemaType.c_type() is a bit awkward: it takes two optional
boolean flags is_param and is_unboxed, and they should never both
be True.

Add a new method for each of the flags, and drop the flags from
c_type().

Most callers pass no flags; they remain unchanged.

One caller passes is_param=True; call the new .c_param_type()
instead.

One caller passes is_unboxed=True, except for simple union types.
This is actually an ugly special case that will go away soon, so
until then, we now have to call either .c_type() or the new
.c_unboxed_type().  Tolerable in the interim.

It requires slightly more Python, but is arguably easier to read.
Suggested-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NEric Blake <eblake@redhat.com>
Message-Id: <1458254921-17042-4-git-send-email-eblake@redhat.com>
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>