vhost_user_devices: net: Add MTU feature flag
vhost_user_devices: net: Add MTU feature flag

Since VIRTIO_NET_F_MTU was implemented by crrev.com/c/2917620, we can
add the feature flag.

BUG=none
TEST=build

Change-Id: If7190d5b3fecaab7f9ba152562149fa70a22d1d9
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/2964520
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Stephen Barber <7719587e4b5a49340661b214ec85602f3918d06f@chromium.org>
Reviewed-by: Chirantan Ekbote <35787c0e27de5c48735e07f8324823b765e8bcbc@chromium.org>
Commit-Queue: Keiichi Watanabe <815e4e8f7c92a23424d7c3f51ead622bcedbc720@chromium.org>

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