Originally we kept snapshots under /var/lib/lxcsnaps.  If a
separate btrfs is mounted at /var/lib/lxc, then we can't
make btrfs snapshots under /var/lib/lxcsnaps.

This patch moves the default directory to /var/lib/lxc/lxcsnaps.
If /var/lib/lxcsnaps already exists, then use that.  Don't allow
any container to be used with the name 'lxcsnaps'.
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>