devices: virtio: add peek operation to queue
devices: virtio: add peek operation to queue

This allows the caller to grab a buffer without committing to using it,
which can be used in the case where two resources (a virtio buffer plus
some other resource) need to be acquired simultaneously.

BUG=None
TEST=build_test.py

Change-Id: Icb61de99db807648ff02c41f95b3128ecce41501
Signed-off-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/1904638
Reviewed-by: Stephen Barber <7719587e4b5a49340661b214ec85602f3918d06f@chromium.org>
Tested-by: Stephen Barber <7719587e4b5a49340661b214ec85602f3918d06f@chromium.org>
Commit-Queue: Stephen Barber <7719587e4b5a49340661b214ec85602f3918d06f@chromium.org>

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