If we have empty pagemap, the page server receives no
IO-s for one and doesn't create image file. Intoroduce
the OPEN command to say to page server, that we're about
to xfer and image so it can prepare.

https://bugzilla.openvz.org/show_bug.cgi?id=2584Signed-off-by: Pavel Emelyanov <xemul@parallels.com>