virgl/vtest: receive and handle shared memory fd

virgl/vtest: receive and handle shared memory fd

The only tricky part is with protocol 0 we can either have
a display target or resource backing store.  With protocol
2 we can have both.  Make the map/unmap functions only deal
with the resource backing store.

v2: Handle MSAA texture case.
v3: spelling
v4: Fix dangling else (@prak)
v5: mmap --> os_mmap (@prak) + added comments (@gerddie)

Signed-off-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-By: Gert Wollny <gert.wollny@collabora.com>
Reviewed-By: Piotr Rak <p.rak@samsung.com>