virtio: video: call set_output_buffer_count() just before passing the first buffer
virtio: video: call set_output_buffer_count() just before passing the first buffer

GAVDA checks if the client sends any buffer between 2
AssignPictureBuffers() call. To guarantee crosvm always passes buffers
after AssignPictureBuffers() call, this CL moves the call just before
passing the first buffer to VDA session.

BUG=b:168557465
TEST=run android.media.cts.AdaptivePlaybackTest#testVP8_adaptiveDrcEarlyEos
     and not see GAVDA's error log mentioned at commit message

Change-Id: I777ff914688ea589a4cf8921fc05ad59d3161b5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/2463052
Tested-by: Chih-Yu Huang <f815e8cb2f117b8947610323682cb280404b8de3@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Alexandre Courbot <301cd6a3ea50a894b4b51e385c5eb12b9a4ffc0c@chromium.org>
Commit-Queue: Chih-Yu Huang <f815e8cb2f117b8947610323682cb280404b8de3@chromium.org>

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