Allow user to manually override enable_vabc_xor

--enable_vabc_xor=false to disable, --enable_vabc_xor=true or
--enable_vabc_xor to enable

Also the flag is now enabled by default. But update_engine will
still ignore XOR ops, as update_engine support for XOR is WIP.

Test: th
Change-Id: I038b0dda5cc9efcc1620e72c858626b56ffb1611