sockets: use weak aliases instead of qemu-tool.c

qemu-tool.c has its own (largeish) set of dependencies.  Weak aliases
can be placed directly where people use them, and do not contribute
to increasing the dependencies of generic utility files.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

qemu-sockets.c

@@ -967,3 +967,10 @@ int socket_init(void)
 #endif
     return 0;
 }
+
+static int default_monitor_get_fd(Monitor *mon, const char *name, Error **errp)
+{
+    error_setg(errp, "only QEMU supports file descriptor passing");
+    return -1;
+}
+QEMU_WEAK_ALIAS(monitor_get_fd, default_monitor_get_fd);

qemu-tool.c

@@ -38,12 +38,6 @@ const char *qemu_get_vm_name(void)
 
 Monitor *cur_mon;
 
-int monitor_get_fd(Monitor *mon, const char *name, Error **errp)
-{
-    error_setg(errp, "only QEMU supports file descriptor passing");
-    return -1;
-}
-
 void vm_stop(RunState state)
 {
     abort();