parallels: wrong call to bdrv_truncate

Parallels driver should not call bdrv_truncate if the image was opened
in the read-only mode. Without the patch
    qemu-img check harddisk.hds
asserts with
    bdrv_truncate: Assertion `child->perm & BLK_PERM_RESIZE' failed.

Parameters used on the write path are not needed if the image is opened
in the read-only mode.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Reported-by: Edgar Kaziahmedov <edos@virtuozzo.mipt.ru>
Message-id: 1490625488-7980-1-git-send-email-den@openvz.org
CC: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
