ploop: implement read-only mount
ploop: implement read-only mount

Also, fix snapshot invalidation which is quite important.

Signed-off-by: Kir Kolyshkin <c0159612b0ef32139322233232df1a0d189c4b9b@openvz.org>

kolyshkin/mosaic,xemul/mosaic,xemul/mosaic,xemul/mosaic,kolyshkin/mosaic,kolyshkin/mosaic