vhost: actually track log eventfd file

While reviewing vhost log code, I found out that log_file is never
set. Note: I haven't tested the change (QEMU doesn't use LOG_FD yet).

Cc: [e-mail]
Signed-off-by: Marc-André Lureau <[e-mail]>
Signed-off-by: Michael S. Tsirkin <[e-mail]>

drivers/vhost/vhost.c

···
 		}
 		if (eventfp != d->log_file) {
 			filep = d->log_file;
+			d->log_file = eventfp;
 			ctx = d->log_ctx;
 			d->log_ctx = eventfp ?
 				eventfd_ctx_fileget(eventfp) : NULL;