VMM/handler: Use more common.lisdir() in Handler's methods _chkenv,
_delete_domain_dir, _delete_home, _get_disk_usage, _make_domain_dir,
and _make_home.
Handler._make_home(): Also check if the domain directory exists.
.. Substitutions in section 1
.. |account.disk_usage| replace:: `account.disk_usage`
.. _account.disk_usage: vmm.cfg.5#account-disk-usage
.. |domain.force_deletion| replace:: `domain.force_deletion`
.. _domain.force_deletion: vmm.cfg.5#domain-force-deletion
.. |misc.transport| replace:: `misc.transport`
.. _misc.transport: vmm.cfg.5#misc-transport
.. #EOF