xen-netback: allow changing the MAC address of the interface

xen-netback: allow changing the MAC address of the interface

Sometimes it is useful to be able to change the MAC address of the
interface for netback devices. For example, when using ebtables it may
be useful to be able to distinguish traffic from different interfaces
without depending on the interface name.

Reported-by: Nikita Borzykh <[email protected]>
Reported-by: Paul Harvey <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: Konrad Rzeszutek Wilk <[email protected]>
Acked-by: Ian Campbell <[email protected]>
Signed-off-by: Matt Wilson <[email protected]>
Reviewed-by: Konrad Rzeszutek Wilk <[email protected]>
Signed-off-by: David S. Miller <[email protected]>