virtio-gpu: add VIRTIO_GPU_F_EDID feature

The feature allows the guest request an EDID blob (describing monitor
capabilities) for a given scanout (aka virtual monitor connector).

It brings a new command message, which has just a scanout field (beside
the standard virtio-gpu header) and a response message which carries the
EDID data.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20181030063206.19528-2-kraxel@redhat.com