monitor: report entirety of hmp command on error

When a user incorrectly provides an hmp command, an error response will be
printed that prompts the user to try "help <command name>". However, when
the command contains multiple parts e.g. "info uuid xyz", only the last
whitespace delimited string will be reported (in this example "info" will
be dropped and the message will read "Try "help uuid" for more information",
which is incorrect).

Let's correct this by capturing the entirety of the command from the command
line -- excluding any extraneous characters.

Reported-by: Mikhail Fokin <fokin@de.ibm.com>
Signed-off-by: Collin Walling <walling@linux.ibm.com>
Message-Id: <ee680f5e-ac9a-479d-f65e-9f8ae9cfe5d4@linux.ibm.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

monitor.c

@@ -3431,6 +3431,7 @@ static void handle_hmp_command(Monitor *mon, const char *cmdline)
 {
     QDict *qdict;
     const mon_cmd_t *cmd;
+    const char *cmd_start = cmdline;
 
     trace_handle_hmp_command(mon, cmdline);
 
@@ -3447,8 +3448,11 @@ static void handle_hmp_command(Monitor *mon, const char *cmdline)
 
     qdict = monitor_parse_arguments(mon, &cmdline, cmd);
     if (!qdict) {
-        monitor_printf(mon, "Try \"help %s\" for more information\n",
-                       cmd->name);
+        while (cmdline > cmd_start && qemu_isspace(cmdline[-1])) {
+            cmdline--;
+        }
+        monitor_printf(mon, "Try \"help %.*s\" for more information\n",
+                       (int)(cmdline - cmd_start), cmd_start);
         return;
     }