Implement VIRTIO_GPU_F_MEMORY

Land TTM removal, and and use drm_gem_shmem_helper for guest memory heap, and drm_gem_vram_helper for host-mapped memory. Four new ioctls, approximately:

1. drm_virtgpu_send_metadata_request (should this use another virtio-gpu queue?)
2. drm_virtgpu_get_metadata_response
3. drm_virtgpu_resource_create_v2
4. drm_virtgpu_resource_sync (more or less emulates DMA_BUF_IOCTL_SYNC). Should return fence so consumers can wait on it.

We probably don't want to expose any new APIs until we have Vulkan passing conformance (may take up to a year, at-least). However, we can do in-tree development on drm-misc-next. @kraxel, may I assign this to you?