vmm_vhost: Remove SlaveReqHandler type parameter in SlaveReqHelper
vmm_vhost: Remove SlaveReqHandler type parameter in SlaveReqHelper

The type parameter `S: SlaveReqHandler` for `SlaveReqHelper` was
introduced to check if it's vvu or not.
Howver, SlaveReqHelper shouldn't take a type parameter of
`SlaveReqHandler` because the helper will be used in the VVU proxy
device which doesn't  have its own req_handler.

So, instead of taking the type parameter, we should have a new bool
field which indicates if it's VVU or not.

BUG=b:194136484
TEST=cargo test --all-features in vmm_vhost
TEST=kokoro

Change-Id: I35f20ef7c42cd74678ab5d87b68bbb58586d2fa5
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3320122
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Chirantan Ekbote <35787c0e27de5c48735e07f8324823b765e8bcbc@chromium.org>
Commit-Queue: Keiichi Watanabe <815e4e8f7c92a23424d7c3f51ead622bcedbc720@chromium.org>

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