merge biscotti-4.18 into machina-4.18

This integrates support for virtio-wayland and virtio-magma from the
biscotti-4.18 branch. It also merges biscotti_defconfig into
machina_defconfig, deleting the former.

Test: ran guest_integration_tests
Change-Id: I19b9df0e5766526f93dc0b89be6cc8fdb025be9e