nmdm is a FreeBSD driver which allows to create a pair of tty
devices one of which is passed to the guest and second is used
by the client.

This patch adds new 'nmdm' character device type. Its definition
looks this way:

<serial type='nmdm'>
  <source master='/dev/nmdm0A' slave='/dev/nmdm0B'/>
</serial>

Master is passed to the hypervisior and slave is used for client
connection.

Also implement domainOpenConsole() for bhyve driver based on that.