criu: Rename private fields

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
criu: Close all fds before going to criu service

When starting swrk in Popen we need to close all descriptors
except for the controlling one and memfd. Unfortunatelly python
doesn't allow to keep arbitrary descriptors, so redirect the
former one into 0 and the latter into 1.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
phaul: Open only one connection to criu

With the new keep_open we may now stat several page servers
in one connection and terminate it with restore req.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
memsk: Move mem_sk into criu_api class

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>