vios_backend: Lock once and receive a single packet per command
vios_backend: Lock once and receive a single packet per command

BUG=b:174713663

Change-Id: I7bae76f01464492fb43540f7d322ce58304d44c3
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/2993287
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Auto-Submit: Jorge Moreira Broche <dbc2f9dc0b0e21f6bd297b0a2dc4733f06f8a572@google.com>
Commit-Queue: Jorge Moreira Broche <dbc2f9dc0b0e21f6bd297b0a2dc4733f06f8a572@google.com>
Reviewed-by: Chirantan Ekbote <35787c0e27de5c48735e07f8324823b765e8bcbc@chromium.org>

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