qmp: convert blockdev-snapshot-sync to a wrapper around transactions

Simplify the blockdev-snapshot-sync code and gain failsafe operation
by turning it into a wrapper around the new transaction command.  A new
option is also added matching "mode".

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
diff --git a/hmp-commands.hx b/hmp-commands.hx
index ed88877..6980214 100644
--- a/hmp-commands.hx
+++ b/hmp-commands.hx
@@ -882,14 +882,17 @@
 
     {
         .name       = "snapshot_blkdev",
-        .args_type  = "device:B,snapshot-file:s?,format:s?",
-        .params     = "device [new-image-file] [format]",
+        .args_type  = "reuse:-n,device:B,snapshot-file:s?,format:s?",
+        .params     = "[-n] 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"
                       "new image file will become the new root image.\n\t\t\t"
                       "If format is specified, the snapshot file will\n\t\t\t"
                       "be created in that format. Otherwise the\n\t\t\t"
-                      "snapshot will be internal! (currently unsupported)",
+                      "snapshot will be internal! (currently unsupported).\n\t\t\t"
+                      "The default format is qcow2.  The -n flag requests QEMU\n\t\t\t"
+                      "to reuse the image found in new-image-file, instead of\n\t\t\t"
+                      "recreating it from scratch.",
         .mhandler.cmd = hmp_snapshot_blkdev,
     },