devices: virtio: video: move session creation to its own method
devices: virtio: video: move session creation to its own method

This makes the code a bit clearer.

BUG=None
TEST=Video plays in Youtube.

Change-Id: Ia904e693b692d70949afd529218dfc0aed7ec1b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/2409490
Tested-by: Alexandre Courbot <301cd6a3ea50a894b4b51e385c5eb12b9a4ffc0c@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Auto-Submit: Alexandre Courbot <301cd6a3ea50a894b4b51e385c5eb12b9a4ffc0c@chromium.org>
Reviewed-by: Keiichi Watanabe <815e4e8f7c92a23424d7c3f51ead622bcedbc720@chromium.org>
Commit-Queue: Alexandre Courbot <301cd6a3ea50a894b4b51e385c5eb12b9a4ffc0c@chromium.org>

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