cros_vm_lib: use virtio-gpu if supported

VM images for most boards only support cirrus, in particular when the
kernel is 3.18 or older.
Some boards, particularly the ones dedicated to VM targets, do support
virtio-gpu and are expected to use it to avoid being limited to 800x600
resolution.

betty, newbie and novato support virtio-gpu and their use case requires
it, so make them use virtio-gpu to catch regressions.

amd64-generic is not specifically a VM target but use virtio-gpu anyway
to provide a simple test case for external contributors.

BUG=chromium:709712
BUG=chromium:741886
TEST=./bin/cros_start_vm --board=novato starts qemu with '-vga virtio'
TEST=./bin/cros_start_vm --board=cyan starts qemu with '-vga cirrus'

Change-Id: I2f67347fde71c3cbfb25f5820d7df72daa936a4a
Reviewed-on: https://chromium-review.googlesource.com/572042
Commit-Ready: Nicolas Norvez <norvez@chromium.org>
Tested-by: Nicolas Norvez <norvez@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
1 file changed