QMP: add snapshot-blkdev-sync command

Add QMP bits for snapshot_blkdev command. This is the same as
snapshot_blkdev in the human monitor. The command is synchronous.

In the future async commands and or a break down of the functionality
into multiple commands might be added.

Also change the 'snapshot_file' argument to 'snapshot-file' in
the human monitor, so that it matches QMP.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@gmail.com>
diff --git a/blockdev.c b/blockdev.c
index a97a801..0b8d3a4 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -568,7 +568,7 @@
 int do_snapshot_blkdev(Monitor *mon, const QDict *qdict, QObject **ret_data)
 {
     const char *device = qdict_get_str(qdict, "device");
-    const char *filename = qdict_get_try_str(qdict, "snapshot_file");
+    const char *filename = qdict_get_try_str(qdict, "snapshot-file");
     const char *format = qdict_get_try_str(qdict, "format");
     BlockDriverState *bs;
     BlockDriver *drv, *old_drv, *proto_drv;
@@ -577,7 +577,7 @@
     char old_filename[1024];
 
     if (!filename) {
-        qerror_report(QERR_MISSING_PARAMETER, "snapshot_file");
+        qerror_report(QERR_MISSING_PARAMETER, "snapshot-file");
         ret = -1;
         goto out;
     }