cros_vm_lib: always use virtio_gpu

cirrus support has been removed, virtio_gpu is the only supported
option.
This fixes cros_vm_test which is failing because the board is not
specified by the chromite calling code, so qemu uses cirrus by default.
This in turns breaks Chrome, so system services (e.g. update_engine)
aren't started.

BUG=chromium:779267
BUG=chromium:782052
BUG=chromium:710629
TEST=cros_vm_test on novato passes

Change-Id: I87b1e03e8cc60169ad9bc47433c4b270233c60e8
Reviewed-on: https://chromium-review.googlesource.com/761416
Commit-Ready: Nicolas Norvez <norvez@chromium.org>
Tested-by: Nicolas Norvez <norvez@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
1 file changed