devices: virtio: console: move process_transmit_request around
devices: virtio: console: move process_transmit_request around

Move this function to a place closer from its call site - this will make
the following CL less cluttered.

BUG=None
TEST=cargo build

Change-Id: I2837af9bb1f721967d7b7676140a8bd11973a34b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3600171
Commit-Queue: Alexandre Courbot <301cd6a3ea50a894b4b51e385c5eb12b9a4ffc0c@chromium.org>
Reviewed-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Reviewed-by: Keiichi Watanabe <815e4e8f7c92a23424d7c3f51ead622bcedbc720@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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