qapi: Permit alternates with just one branch

qapi: Permit alternates with just one branch

A union or alternate without branches makes no sense and doesn't work:
it can't be instantiated.  A union or alternate with just one branch
works, but is degenerate.  We accept the former, but reject the
latter.  Weird.  docs/devel/qapi-code-gen.txt doesn't mention the
difference.  It claims an alternate definition is "is similar to a
simple union type".

Permit degenerate alternates to make them consistent with unions.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190913201349.24332-10-armbru@redhat.com>