virtio-gpu api: v2 resources

This patch adds a new virtio feature for v2 (unformated) resources.

v2 resources emulate GEM create, by passing down guest arguments
straight to the host, while retaining arguments needed by the
kernel (memory heap, caching) and virtual machine manager (sg-list,
resource sharing)

guest/host interface:  Three new commands are added:
 (1) One command to create v2 resources
 (2) command to scanout v2 resources (we have to specify the format for
     scanout here).
 (3) One command to synchronize v2 resources. Inspired by
     vkFlushMappedMemoryRanges / vkInvalidateMappedMemoryRanges. Returns an
     out fence -- Android userspace an actually benefit from that given
     the (*unlockAsync) gralloc API

kernel/userspace interface: TBD.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>