vmdk: Add trivial backing_fmt support

vmdk already requires that if backing_file is present, that it be
another vmdk image (see vmdk_co_do_create).  Meanwhile, we want to
move towards always being explicit about the backing format for other
drivers where it matters.  So for convenience, make qemu-img create -F
vmdk work, while rejecting all other explicit formats (note that this
is only for QemuOpts usage; there is no change to the QAPI to allow a
format through -blockdev).

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200706203954.341758-5-eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/vmdk.c

@@ -2633,6 +2633,14 @@ static int coroutine_fn vmdk_co_create_opts(BlockDriver *drv,
     bool zeroed_grain;
     bool compat6;
     VMDKCreateOptsData data;
+    char *backing_fmt = NULL;
+
+    backing_fmt = qemu_opt_get_del(opts, BLOCK_OPT_BACKING_FMT);
+    if (backing_fmt && strcmp(backing_fmt, "vmdk") != 0) {
+        error_setg(errp, "backing_file must be a vmdk image");
+        ret = -EINVAL;
+        goto exit;
+    }
 
     if (filename_decompose(filename, path, prefix, postfix, PATH_MAX, errp)) {
         ret = -EINVAL;
@@ -2691,6 +2699,7 @@ static int coroutine_fn vmdk_co_create_opts(BlockDriver *drv,
                             vmdk_co_create_opts_cb, &data, errp);
 
 exit:
+    g_free(backing_fmt);
     g_free(adapter_type);
     g_free(backing_file);
     g_free(hw_version);
@@ -3026,6 +3035,11 @@ static QemuOptsList vmdk_create_opts = {
             .type = QEMU_OPT_STRING,
             .help = "File name of a base image"
         },
+        {
+            .name = BLOCK_OPT_BACKING_FMT,
+            .type = QEMU_OPT_STRING,
+            .help = "Must be 'vmdk' if present",
+        },
         {
             .name = BLOCK_OPT_COMPAT6,
             .type = QEMU_OPT_BOOL,