VMM/domain: Enabled transport-validation.
Validate the transport also in Domain.set_transport() and
Domain.update_transport().
# All parameters are described in pgsql_table(5) / PGSQL PARAMETERS
#
# The hosts that Postfix will try to connect to and query from.
hosts = localhost
# The user name and password to log into the pgsql server.
user = postfix
password = some_password
# The database name on the servers.
dbname = mailsys
# The SQL query template used to search the database
query = SELECT transport FROM postfix_transport_map('%u', '%d')