This patch adds a new ->migrate API call with three commands:

MIGRATE_DUMP: this is basically just ->checkpoint()
MIGRATE_RESTORE: this is just ->restore()
MIGRATE_PRE_DUMP: this can be used to invoke criu's pre-dump command on the
    container.

A small addition to the (pre-)dump commands is the ability to specify a
previous partial dump directory, so that one can use a pre-dump of a
container.

Finally, this new API call uses a structure to pass options so that it can
be easily extended in the future (e.g. to CRIU's --leave-frozen option in
the future, for potentially smarter failure handling on restore).

v2: remember to flip the return code for legacy ->checkpoint and ->restore
    calls
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>