parasite: send PARASITE_CMD_FINI before resuming the target process

The control socket has enough buffer for one command and the target
process will not wait a new command, so we will avoid extra context
switches.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>