monitor: avoid use of global *cur_mon in monitor_find_completion()

Parameter *mon is added, and local variable *mon added in previous patch
is removed. The caller readline_completion(), pass rs->mon as value, which
should be initialized in readline_init() called by monitor_init().

Signed-off-by: Wenchao Xia <xiawenc@linux.vnet.ibm.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

include/monitor/readline.h

@@ -8,7 +8,8 @@
 #define READLINE_MAX_COMPLETIONS 256
 
 typedef void ReadLineFunc(Monitor *mon, const char *str, void *opaque);
-typedef void ReadLineCompletionFunc(const char *cmdline);
+typedef void ReadLineCompletionFunc(Monitor *mon,
+                                    const char *cmdline);
 
 typedef struct ReadLineState {
     char cmd_buf[READLINE_CMD_BUF_SIZE + 1];