msg_socket: AsyncReceiver doesn't need a mut ref
msg_socket: AsyncReceiver doesn't need a mut ref

The mut reference isn't used, an immutable reference is sufficient.
Switch to make borrowing the receiver easier for users.

Change-Id: Ic8582deb68997de53e5a53730b3909a1c4dcf699
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/2159936
Reviewed-by: Chirantan Ekbote <35787c0e27de5c48735e07f8324823b765e8bcbc@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Tested-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Commit-Queue: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>

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