VMM/password: moved the 'scheme check' code from pwhash() to the
new function verify_scheme().
VMM/Config: use verify_scheme() to check the scheme when
LazyConfig.set() is called.
VMM/Config: removed the default value of misc.dovecot_version.
There are too many different versions installed on different systems.
So, it doesn't make much sense to have a default value.