virtio-gpu: split virtio-gpu, introduce virtio-gpu-base

Add a base class that is common to virtio-gpu and vhost-user-gpu
devices.

The VirtIOGPUBase base class provides common functionalities necessary
for both virtio-gpu and vhost-user-gpu:
- common configuration (max-outputs, initial resolution, flags)
- virtio device initialization, including queue setup
- device pre-conditions checks (iommu)
- migration blocker
- virtio device callbacks
- hooking up to qemu display subsystem
- a few common helper functions to reset the device, retrieve display
informations
- a class callback to unblock the rendering (for GL updates)

What is left to the virtio-gpu subdevice to take care of, in short,
are all the virtio queues handling, command processing and migration.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20190524130946.31736-8-marcandre.lureau@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>