Use the changed `mail_virtual_users` structure to ensure directories and
sieve rules are created for each defined account.

mail_virtual_xx list were not use correctly in tasks.

We got error 'unicode object' has no attribute 'name' because:
with_items:
 - "{{ mail_virtual_xx }}
is understood has a list of strings containing information extracted from {{ mail_virtual_xx }} variable

 Now we use:
   with_items: virtual_domain_xx}}"