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From console

 * /run/lock: creating directory
 * /run/lock: correcting owner
 * Mounting xenfs ... [ ok ]
[    3.798024] random: doveconf: getrandom without GRND_NONBLOCK while crng not ready
[  259.681214] random: crng init done
 * Caching service dependencies ... [ ok ]
 * Remounting /dev according to fstab ... [ ok ]

Note: that GRND_NONBLOCK printout is from a patched kernel

From dovecot init.d script

cfgfile=/etc/dovecot/dovecot${instance:+.$instance}.conf
pidfile=$(doveconf -c $cfgfile -h base_dir 2>/dev/null)/master.pid
command=/usr/sbin/dovecot
command_args=${instance:+-c $cfgfile}

Commit that introduced the fault commit:7307d5b5

kernel patch used to find what cases the boot hang:

<@@ -2055,6 +2055,9 @@
        if (!crng_ready()) {
                if (flags & GRND_NONBLOCK)
                        return -EAGAIN;
+               printk(KERN_NOTICE "random: %s: getrandom without "
+                       "GRND_NONBLOCK while crng not ready\n",
+                       current->comm);
                ret = wait_for_random_bytes();
                if (unlikely(ret))
                        return ret;

(from redmine: issue id 10320, created on 2019-04-23)