qapi: Visit variants in visit_type_FOO_fields()

qapi: Visit variants in visit_type_FOO_fields()

We initially created the static visit_type_FOO_fields() helper
function for reuse of code - we have cases where the initial
setup for a visit has different allocation (depending on whether
the fields represent a stand-alone type or are embedded as part
of a larger type), but where the actual field visits are
identical once a pointer is available.

Up until the previous patch, visit_type_FOO_fields() was only
used for structs (no variants), so it was covering every field
for each type where it was emitted.

Meanwhile, the code for visiting unions looks like:

static visit_type_U_fields() {
    visit base;
    visit local_members;
}
visit_type_U() {
    visit_start_struct();
    visit_type_U_fields();
    visit variants;
    visit_end_struct();
}

which splits the fields of the union visit across two functions.
Move the code to visit variants to live inside visit_type_U_fields(),
while making it conditional on having variants so that all other
instances of the helper function remain unchanged.  This is also
a step closer towards unifying struct and union visits, and towards
allowing one union type to be the branch of another flat union.

The resulting diff to the generated code is a bit hard to read,
but it can be verified that it touches only union types, and that
the end result is the following general structure:

static visit_type_U_fields() {
    visit base;
    visit local_members;
    visit variants;
}
visit_type_U() {
    visit_start_struct();
    visit_type_U_fields();
    visit_end_struct();
}

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1455778109-6278-7-git-send-email-eblake@redhat.com>
[gen_visit_struct_fields() parameter variants made mandatory]
Signed-off-by: Markus Armbruster <armbru@redhat.com>