c/r: check for criu images in the checkpoint directory

c/r: check for criu images in the checkpoint directory

CRIU can get confused if there are two dumps that are written to the same
directory, so we make some minimal effort to prevent people from doing this.
This is a better alternative than forcing liblxc to create the directory, since
it is mostly race free (and neither solution is bullet proof anyway if someone
rsyncs some bad images over the top of the good ones).

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>