lsmcli: Fix bug of volume-masking '--init' option.

 * Volume mask will fail with '--init' option[1].
 * Update simulator plugin to support creating access group when volume mask.
 * Add a test into cmdtest.py for this.
 * Manpage updated to guide targetd user.

[1] It's the only way to mask volume to new WWPN/iSCSI IQN on targetd.

Signed-off-by: Gris Ge <fge@redhat.com>