configure: Provide more helpful message if libvte not present

If the system has GTK but not libvte, it's more helpful to
tell the user that libvte is missing than to simply say that
GTK is not present.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1374162121-31582-1-git-send-email-peter.maydell@linaro.org
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>