devices: fix build with audio feature disabled
devices: fix build with audio feature disabled

When building without the "audio" feature, an import of virtio::snd in
pci_device failed to compile; wrap it in a feature check to fix this.
This fixes the crosvm fuzzer build.

BUG=chromium:1174254
TEST=cargo build --no-default-features
TEST=build crosvm with amd64-generic asan profile

Change-Id: Iecd549c313d6210ee90e9e405c02a9e7581e1141
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/2674260
Tested-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Auto-Submit: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Reviewed-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Reviewed-by: Manoj Gupta <1d49d0f302d724a1dc5fc4d717d15418e9808ddd@chromium.org>
Commit-Queue: Manoj Gupta <1d49d0f302d724a1dc5fc4d717d15418e9808ddd@chromium.org>

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