lxc-checkpoint: automatically detect if --external or --veth-pair

lxc-checkpoint: automatically detect if --external or --veth-pair

With the criu release 2.8 criu deprecated the --veth-pair command-line
option in favor of --external:

f2037e6 veth: Make --external support --veth-pair

git tag --contains f2037e6d3445fc400
v2.8

With this commit lxc-checkpoint will automatically switch between
the new and old command-line option dependent on the detected
criu version.

For criu version older than 2.8 something like this will be used:

  --veth-pair eth0=vethYOK6RW@lxcbr0

and starting with criu version 2.8 it will look like this:

  --external veth[eth0]:vethCRPEYL@lxcbr0

Signed-off-by: Adrian Reber <areber@redhat.com>