Change snapshot_blkdev hmp to use correct argument type for device

Pointed out by Markus

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

hmp-commands.hx

@@ -822,7 +822,7 @@ ETEXI
 
     {
         .name       = "snapshot_blkdev",
-        .args_type  = "device:s,snapshot_file:s?,format:s?",
+        .args_type  = "device:B,snapshot_file:s?,format:s?",
         .params     = "device [new-image-file] [format]",
         .help       = "initiates a live snapshot\n\t\t\t"
                       "of device. If a new image file is specified, the\n\t\t\t"