vga: enable virtio test, fix ddc oob read

# gpg: Signature made Fri 11 Jan 2019 13:58:12 GMT
# gpg:                using RSA key 4CB6D8EED3E87138
# gpg: Good signature from "Gerd Hoffmann (work) <kraxel@redhat.com>"
# gpg:                 aka "Gerd Hoffmann <gerd@kraxel.org>"
# gpg:                 aka "Gerd Hoffmann (private) <kraxel@gmail.com>"
# Primary key fingerprint: A032 8CFF B93A 17A7 9901  FE7D 4CB6 D8EE D3E8 7138

* remotes/kraxel/tags/vga-20190111-pull-request:
  i2c-ddc: fix oob read
  tests/display-vga: Enable virtio-vga test
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>