This is more strict than VPP itself, which does allow a destination to
be L3 while the source is L2:
DBGvpp# show mode
l3 local0
l3 GigabitEthernet3/0/0
l3 GigabitEthernet3/0/1
l3 HundredGigabitEthernet12/0/0
l3 HundredGigabitEthernet12/0/1
DBGvpp# set interface l2 xconnect GigabitEthernet3/0/0 GigabitEthernet3/0/1
DBGvpp# show mode
l3 local0
l2 xconnect GigabitEthernet3/0/0 GigabitEthernet3/0/1
l3 GigabitEthernet3/0/1
l3 HundredGigabitEthernet12/0/0
l3 HundredGigabitEthernet12/0/1
But it seems undesirable to have Gi3/0/1 be in l3 mode even though it's
the target of a cross connect, so I'm going to leave the behavior
stricter than VPP.