Ensure remote daemon unions are always non-zero length

If the remote daemon args/ret unions ever become zero length
(due to a build / Makefile bug) then bad stuff happens at
runtime. Add a compile time assertion to check for this kind
of problem

* daemon/remote.h: Ensure non-zero length unions

daemon/remote.h

@@ -30,18 +30,22 @@
 typedef union {
 # include "remote_dispatch_args.h"
 } dispatch_args;
+verify(sizeof(dispatch_args) > 0);
 
 typedef union {
 # include "remote_dispatch_ret.h"
 } dispatch_ret;
+verify(sizeof(dispatch_ret) > 0);
 
 typedef union {
 # include "qemu_dispatch_args.h"
 } qemu_dispatch_args;
+verify(sizeof(qemu_dispatch_args) > 0);
 
 typedef union {
 # include "qemu_dispatch_ret.h"
 } qemu_dispatch_ret;
+verify(sizeof(qemu_dispatch_ret) > 0);