virtio: vhost: user: remove Sized constraint on VhostUserBackend trait
virtio: vhost: user: remove Sized constraint on VhostUserBackend trait

This constraint seems to be unneeded and prevents us from creating
VhostUserBackend trait objects.

BUG=b:217480043
TEST=cargo build

Change-Id: I79c1015d3a45902bc18cd22e52de78b2639354aa
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3702777
Reviewed-by: Keiichi Watanabe <815e4e8f7c92a23424d7c3f51ead622bcedbc720@chromium.org>
Commit-Queue: Alexandre Courbot <301cd6a3ea50a894b4b51e385c5eb12b9a4ffc0c@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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