Add an option to set Ethernet link addresses for VM

add lines like:

vm_link_0="AA:BB:CC:DD:EE:FF"

If this is not present or blank then just use the random link
address generated by tap/vtnet driver.
Add extra settings with altered numeric suffix (e.g.vm_link_4)
to change the link address of other virtual network devices.

This is primarily for setting IP via DHCP.

Signed-off-by: Christopher Hall <hsw@bitmark.com>