Allow editing the VM mac address

In order to give the user control of IP address assignment in DHCP
environments it is important to allow the MAC address to be specified
explicitly.  Make MAC address a customizable parameter while still
suggesting a default value for users who may not care.

Change-Id: Ie75af322fc403809e0b044eb13561e6d16e1c4d4
Signed-off-by: Adam Litke <alitke@redhat.com>
Bug-url: https://bugzilla.redhat.com/show_bug.cgi?id=1034706