crit: Fix ipc load/dump for io.BytesIO input/output

There are loads and dumps method in pycriu.images that work with
strings, instead of open()-ed files. For simplicity strings are
turned into streams with io.BytesIO and the files are then pushed
into regular load/dump methods.

The problem is that array.array object doesn't work with io object
in .fromfile/.tofile methods, so we have to read/write the data
from them explicitly and use .fromstring/.tostring for arrays.

With this the crit test finally passes :D

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>