monitor: Fix do_commit() argument type

Currently do_commit() argument type is 's' (string), but it
should be 'B' (block), this way 'commit' gets command completion.
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

qemu-monitor.hx

@@ -15,7 +15,7 @@ STEXI
 Show the help for all commands or just for command @var{cmd}.
 ETEXI
 
-    { "commit", "device:s", do_commit,
+    { "commit", "device:B", do_commit,
       "device|all", "commit changes to the disk images (if -snapshot is used) or backing files" },
 STEXI
 @item commit