qemu-img: Print available options with -o ?

This patch adds a small help text to each of the options in the block drivers
which can be displayed by using qemu-img create -f fmt -o ?

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

block/vpc.c

@@ -594,7 +594,11 @@ static void vpc_close(BlockDriverState *bs)
 }
 
 static QEMUOptionParameter vpc_create_options[] = {
-    { "size", OPT_SIZE },
+    {
+        .name = BLOCK_OPT_SIZE,
+        .type = OPT_SIZE,
+        .help = "Virtual disk size"
+    },
     { NULL }
 };