virtio-gpu: Support host-allocated backing storagevirtio-gpu

The virtio-gpu protocol is based on opaque host resources backed by
guest-allocated memory. This requires transferring data from guest
framebuffers to host display device buffers.

When the host supports VIRGL, the GPU is used for both 2D and 3D
rendering, in which case the memory transfer can take the form of a DMA
operation without incurring significant costs. However, when VIRGL
support isn't available, 2D transfers require a CPU memory copy for
every page flip. Display is slowed down to an unusable state.

To solve this problem, extend the virtio-gpu protocol with the ability
for the host to allocate resource backing storage and map it to the
guest. The allocated memory can then be used directly by the host-side
display device, removing the need for memory copy operations.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

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