WIP: vtest memfd

Intended to fix virgl/virglrenderer#66 (closed) and virgl/virglrenderer#35 (closed).

Note my plan is replace VCMD_TRANSFER_PUT2 / VCMD_TRANSFER_GET2 / VCMD_TRANSFER_RESOURCE_CREATE2 with this, rather than create VCMD_TRANSFER_PUT3 / VCMD_TRANSFER_GET3 / VCMD_TRANSFER_RESOURCE_CREATE3, since I believe this was the original intention behind that and it'll lead to less code.

What does everyone think?