Don't take "our" MAC address from the host

When sending frames to the guest over the tap link, we need a source MAC
address.  Currently we take that from the MAC address of the main interface
on the host, but that doesn't actually make much sense:
 * We can't preserve the real MAC address of packets from anywhere
   external so there's no transparency case here
 * In fact, it's confusingly different from how we handle IP addresses:
   whereas we give the guest the same IP as the host, we're making the
   host's MAC the one MAC that the guest *can't* use for itself.
 * We already need a fallback case if the host doesn't have an Ethernet
   like MAC (e.g. if it's connected via a point to point interface, such
   as a wireguard VPN).

Change to just just use an arbitrary fixed MAC address - I've picked
9a:55:9a:55:9a:55.  It's simpler and has the small advantage of making
the fact that passt/pasta is in use typically obvious from guest side
packet dumps.  This can still, of course, be overridden with the -M option.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

util.h

@@ -96,7 +96,6 @@
 #define PORT_IS_EPHEMERAL(port) ((port) >= PORT_EPHEMERAL_MIN)
 
 #define MAC_ZERO		((uint8_t [ETH_ALEN]){ 0 })
-#define MAC_LAA			((uint8_t [ETH_ALEN]){ BIT(1), 0, 0, 0, 0, 0 })
 #define MAC_IS_ZERO(addr)	(!memcmp((addr), MAC_ZERO, ETH_ALEN))
 
 #ifndef __bswap_constant_16