Revert "api: ->save_config() doesn't need to create container dir"

Revert "api: ->save_config() doesn't need to create container dir"

This reverts commit 2fde07c3990fe09925699da5d9a1762eb279c497.

We need to investigate this in more detail but this commit is breaking
LXD, causing startup to fail with:

lxc foo 20211130202833.906 INFO     conf - conf.c:run_script_argv:336 - Executing script "/bin/mount -t shiftfs -o passthrough=3 "/lxc-ci/build/tmp.WemmpzWGYz/go/src/github.com/lxc/lxd/test/tmp.Cli/0To/containers/foo/rootfs" "/lxc-ci/build/tmp.WemmpzWGYz/go/src/github.com/lxc/lxd/test/tmp.Cli/0To/containers/foo/rootfs"" for container "foo"
lxc foo 20211130202833.912 ERROR    conf - conf.c:run_buffer:321 - Script exited with status 32
lxc foo 20211130202833.912 ERROR    conf - conf.c:lxc_setup_rootfs_prepare_root:3947 - Failed to run pre-mount hooks
lxc foo 20211130202833.912 ERROR    conf - conf.c:lxc_setup:4317 - Failed to setup rootfs
lxc foo 20211130202833.912 ERROR    start - start.c:do_start:1275 - Failed to setup container "foo"

Not entirely sure why we're seeing things blow up as the directory
definitely exists (and contains a valid rootfs) but this was caused by
today's liblxc update.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>