parallels: mark driver as supporting CBT

parallels: mark driver as supporting CBT

Parallels driver indeed support Parallels Dirty Bitmap Feature in
read-only mode. The patch adds bdrv_supports_persistent_dirty_bitmap()
callback which always return 1 to indicate that.

This will allow to copy CBT from Parallels image with qemu-img.

Note: read-write support is signalled through
bdrv_co_can_store_new_dirty_bitmap() and is different.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Reviewed-by: Alexander Ivanov <alexander.ivanov@virtuozzo.com>