build: move coroutine backend selection to meson

To simplify the code, rename coroutine-win32.c to match the option
passed to configure.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

util/meson.build

@@ -78,7 +78,7 @@ if have_block or have_ga
   util_ss.add(files('base64.c'))
   util_ss.add(files('main-loop.c'))
   util_ss.add(files('qemu-coroutine.c', 'qemu-coroutine-lock.c', 'qemu-coroutine-io.c'))
-  util_ss.add(files('coroutine-@0@.c'.format(config_host['CONFIG_COROUTINE_BACKEND'])))
+  util_ss.add(files(f'coroutine-@coroutine_backend@.c'))
   util_ss.add(files('thread-pool.c', 'qemu-timer.c'))
   util_ss.add(files('qemu-sockets.c'))
 endif