Add a the new qemu_create_displaysurface_pixman function, to create
a DisplaySurface backed by an existing pixman image.  In that case
there is no need to create a new pixman image pointing to the same
backing storage.  We can just use the existing image directly.

This does not only simplify things a bit, but most importantly it
gets the reference counting right, so the backing storage for the
pixman image wouldn't be released underneath us.

Use new function in virtio-gpu, where using it actually fixes
use-after-free crashes.

Cc: qemu-stable@nongnu.org
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Message-id: 1459499240-742-1-git-send-email-kraxel@redhat.com