qga: Flatten simple union GuestDeviceId

qga: Flatten simple union GuestDeviceId

Simple unions are simpler than flat unions in the schema, but more
complicated in C and on the QMP wire: there's extra indirection in C
and extra nesting on the wire, both pointless.  They should be avoided
in new code.

GuestDeviceId was recently added for guest-get-devices.  Convert it to
a flat union.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Michael Roth <michael.roth@amd.com>