virgl: introduce transfer queue

virgl: introduce transfer queue

Transfers will be placed here at unmap time instead of incurring
a VM exit. There's an attempt to deduplicate intersecting 1D transfers,
which are surprisingly common.

This can also help with mipmapped texture upload and smaller
textures, where the majority of the time is spent in the guest
kernel / QEMU -- not virglrenderer.  This is shown by the GLbench
texture upload benchmark:

Before:
    texture_upload_rgba_teximage2d_32 = 64.23 mtexel_sec
After:
    texture_upload_rgba_teximage2d_32 = 367.44 mtexel_sec

v2: Split up list iteration functions (@gerddie)
v3: Support for optimizing glBufferSubData
Reviewed-by: Gert Wollny <gert.wollny@collabora.com>