qapi: Move gen_enum(), gen_enum_lookup() back to qapi/types.py

qapi: Move gen_enum(), gen_enum_lookup() back to qapi/types.py

The next commit will split up qapi/common.py.  gen_enum() needs
QAPISchemaEnumMember, and that's in the way.  Move it to qapi/types.py
along with its buddy gen_enum_lookup().

Permit me a short a digression on history: how did gen_enum() end up
in qapi/common.py?  Commit 21cd70dfc1 "qapi script: add event support"
duplicated qapi-types.py's gen_enum() and gen_enum_lookup() in
qapi-event.py.  Simply importing them would have been cleaner, but
wasn't possible as qapi-types.py was a program, not a module.  Commit
efd2eaa6c2 "qapi: De-duplicate enum code generation" de-duplicated by
moving them to qapi.py, which was a module.

Since then, program qapi-types.py has morphed into module types.py.
It's where gen_enum() and gen_enum_lookup() started, and where they
belong.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20191018074345.24034-6-armbru@redhat.com>