bhyve: Set correct mac address chosen by a controller on the VM side.

In order to make DHCP work, each VM has to have a correct mac address set
as dnsmasq service running on each networking bridge answers only to
requests originating with mac addresses chosen by the controller.
bhyve: Fix tap_name method of vif driver by using get() method.

Instead of rising an key error exception, it returns None. It's fully
handled by the get() method of a dictionary.