qcow: require image size to be > 1 for new images

The qcow driver refuses to open images which are less than
2 bytes in size, but will happily create such images. Add
a check in the create path to avoid this discrepancy.
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NAlberto Garcia <berto@igalia.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>
Message-id: 20170623162419.26068-5-berrange@redhat.com
Signed-off-by: NMax Reitz <mreitz@redhat.com>

block/qcow.c

@@ -811,6 +811,12 @@ static int qcow_create(const char *filename, QemuOpts *opts, Error **errp)
     /* Read out options */
     total_size = ROUND_UP(qemu_opt_get_size_del(opts, BLOCK_OPT_SIZE, 0),
                           BDRV_SECTOR_SIZE);
+    if (total_size == 0) {
+        error_setg(errp, "Image size is too small, cannot be zero length");
+        ret = -EINVAL;
+        goto cleanup;
+    }
+
     backing_file = qemu_opt_get_del(opts, BLOCK_OPT_BACKING_FILE);
     if (qemu_opt_get_bool_del(opts, BLOCK_OPT_ENCRYPT, false)) {
         flags |= BLOCK_FLAG_ENCRYPT;