virtio: video: resource: add CPU mapping method
virtio: video: resource: add CPU mapping method

Add a method that allows to conveniently map resources into a linear
buffer, even if the resources are initially sparse (as is often the case
with user memory).

BUG=b:209523781
BUG=b:169295147
TEST=Android Youtube can play videos on Hatch.

Change-Id: If9a9b7764e6522da20d85e2ebca59a77b58573ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3308315
Reviewed-by: Keiichi Watanabe <815e4e8f7c92a23424d7c3f51ead622bcedbc720@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Commit-Queue: Alexandre Courbot <301cd6a3ea50a894b4b51e385c5eb12b9a4ffc0c@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm