For migration optimization features like pre-copy or post-copy migration
the support cannot be determined by simply looking at the CRIU version.
Features like that depend on the architecture/kernel/criu combination
and CRIU offers a feature checking interface to query if it is
supported.

This adds a LXC interface to query CRIU for those feature via the
migrate() API call. For the recent pre-copy migration support in LXD
this can be used to automatically detect if pre-copy migration should be
used.

In addition to the existing migrate() API commands this adds a new
command: 'MIGRATE_FEATURE_CHECK'.

The migrate_opts{} structure is extended by the member features_to_check
which is a bitmask defining which CRIU features should be queried.

Currently only the querying of the features FEATURE_MEM_TRACK and
FEATURE_LAZY_PAGES is supported.
Signed-off-by: Adrian Reber <areber@redhat.com>