bhyve: auto allocate nmdm console paths

bhyve: auto allocate nmdm console paths

Currently, nmdm console device requires user to specify master and slave
path attributes (such as /dev/nmdm0A and /dev/nmdm0B respectively).
However, making user find a non-occupied device name might be not
convenient, especially for the remote connections.

Update the logic to make these attributes optional. In case if not
specified, use /dev/nmdm$UUID[AB], where $UUID is a domain's UUID.
With this schema it's unlikely nmdm device will clash with other domains
or even other non-bhyve nmdm devices.

Signed-off-by: Roman Bogorodskiy <bogorodskiy@gmail.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>