vzmigrate: fix ploop for diff VE_PRIVATE case

Allow vzmigrate a ploop copy when VE private dir on source-host
has a different path as on the target-host. This should fix
bug #2875.

https://bugzilla.openvz.org/2875

[kir@openvz.org: declare vars as locals]

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