api_snapshot: remove lxcapi_snap_open

it wasn't defined, and is dubious in nature.  Yes snapshots are
full containers, but if we encourage using them as such then things
will get messy, i.e. /var/lib/lxcsnaps/c1snaps/c1/snap0.

Pointed out by Stéphane Graber.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
snapshot: rename get_comment to get_commentpath

to better reflect what it is.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>