vhost_user: Modify signature of send_ack_message
vhost_user: Modify signature of send_ack_message

send_ack_message used to take a module local Result as a parameter. It
doesn't make sense for other modules to use this. This change makes the
method take a bool instead of the aforementioned Result.

BUG=b:194136484
TEST=Compile.

Change-Id: I475cc39826db94ef2c686e754e13f478f8109d0a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3285629
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Commit-Queue: Abhishek Bhardwaj <4731196bfa2312dffce2e96e4b970d725db87a47@chromium.org>
Reviewed-by: Keiichi Watanabe <815e4e8f7c92a23424d7c3f51ead622bcedbc720@chromium.org>

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