lsm: add a --lsm-profile flag

In LXD, we use the container name in the LSM profile. If the container name
is changed on migrate (on the host side), we want to use a different LSM
profile name (a. la. --cgroup-root). This flag adds that support.

v2: remove unused field, add comment about double detection in
    kerndat_lsm()

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>