virtio: video: Allow config changes if encoding hasn't started yet.
virtio: video: Allow config changes if encoding hasn't started yet.

When initializing the device for encoding the underlying encoder
session is immediately brought up, as this is required to determine
parameters such as the desired output buffer size. This causes further
configuration changes to be refused, even when the device isn't
encoding anything yet. This CL makes changes to allow configuration
changes by recreating the encoder sessions as long as no buffers have
been queued.

BUG=b:203608233
TEST=tast run DUT arc.VideoEncodeAccel.h264_192p_i420_vm

Change-Id: I7e9d8b41ea2497b5d86aaca7db7ed13af056f11d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3252185
Reviewed-by: Alexandre Courbot <301cd6a3ea50a894b4b51e385c5eb12b9a4ffc0c@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Commit-Queue: David Staessens <a4557d4b9b905b8f9d5c87334b8d9f562e1ec119@chromium.org>

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